Upgrading illumination or running with lights off can be done, but to do so with safeguards against component failure and setting your bike on fire are necessary if drastic changes are made in electrical demand.
Early TWs power the headlight directly off the alternator with AC voltage and no regulation. It is critical to keep about the same running draw as the stock headlight to prevent damaging the charging circuit. If additional tail/clearance lights are desired, switch to LEDs in the stock fixtures to free up wattage for the additional lights. LED conversion of the stock incandescents will free up enough wattage one could add a 10-12 watt LED auxilary headlight to the taillight circuit without problems, and still maintain approximate energy balance. The regulated DC current in the taillight circuit would suit the LED auxilary light to a T, and the light would not need a separate switch as it would be switched same as the taillight. This would be the easiest and simplest way to add a LED auxilary headlight to any TW.
'01 and newer TWs isolate all lighting from the alternator via a rectifier and battery. Therefore, the entire lighting system should be balanced as a whole. This is advantageous because significant wattage can be added to the headlight by replacing the stock incandescent turn/tail/indicator lamps with LEDs while still keeping the system balanced. It is not necessary to provide rectification of AC to DC or attentuation of pulsed DC to run different headlight technologies because Yamaha has already taken care of that.
What it all boils down to is '01 and newer TWs can be easily modded to provide almost double the wattage available for headlights as early models. This is a significant advantage for those of us who ride at night. Newer TWs can enjoy 8-12 times the illumination of the stock early TW headlight when current technology lighting products are used. Those serious about improved lighting with early TWs that develop electrical problems would do well to shop for a complete '01+ electrcial system to retrofit.
Doesn't the '98 use a 35 watt headlight bulb? If so, just dropping in a 55 watt replacement might not be such a good idea. TWs have a balanced electrical system, with alternator ouput about equal to system demand. Mods that move this balance way out of whack can affect electrical components in a bad way. Therefore, replacement lights should leave each circuit with the same total watt draw. 20-25 extra watts of light could overheat your headlight coil. Be careful to keep the system balanced and you can run anything you want. Today there are many lighting options that provide improved intensity and put more light in a useful pattern that draw, singly or in combination, 35 watts or somewhere thereabouts.
For instance, HID coversions are available that draw ~35 watts, and provide 4-6 times the output intensity of a 35 watt incandescent. Unfortunately, early TWs run the headlight on AC current directly off the alternator, and the voltage drops at low rpm. HIDs require DC current, and though they draw low amps, they will not stay lit at low voltage. HIDs are not particularly fond of pulsed DC current at low hertz, so a battery is necessary to attentuate the pulses into a smoother constant DC supply. To run an HID on such a bike it is necessary to use a regulator to convert AC to DC, and a battery to provide sufficient voltage at low engine speed to keep the light on. If the electrical system is modified to meet the needs of the HID system, excellent performance is available.
Current technology LEDs provide about the same intensity of light as HIDs at the same current draw. LEDs generally only light when the current flows one direction, so half the alternator's output is wasted if wired in as a direct replacement for the stock headlight, if the LED lamp is designed to tolerate AC current. If not, it will fry. A rectifier provides pulsed DC output that suits LEDs, no battery required. LEDs will flicker with a pulsed DC current, but at higher rpm you'r eyes will not see the flicker. While not really necessary, a battery will keep LEDs constantly on, rather than flickering, resulting in a more intense output at all engine speeds.
If you want to run in stealth mode, lights off, best to add a regulator to the electrical system that shunts excess alternator output to some sort of heat dissipating balance resistor to deal with the alternator output not being used rather than trying to force it into a charged battery.
The above should provide some ponderable principles of adequately engineering lighting modifications. I leave the development of specific circuit schematics, analysis of replacement and additional lamps, and actual determination of components necessary to safely mod TW lighting systems to the individuals doing the work. Proceed at your own risk
If you saw the schematic planned for Tdub's electrics when she finally dies of old age, you'd freak. LEDs in every fixture, auxilary tail/brake/turns and headlights and navigation system can be shut off to provide wattage for an air compressor, heated grips, handheld spotlight, heated clothing, etc. The entire system is designed to swap one load for another as needed, and the battery is always protected from excessive discharge and charge rates. Even late TWs don't generate enough watts to run everything at once, but they do generate enough power to run everything if thought is put into using what's available to maximum advantage.
I suggest a spreadsheet of stock current draws on each circuit. Make changes as necessary to draw less using more efficient modern lamps, then add accessories to replace the demand at about the same rate the low wattage replacement parts lowered demand. I also suggest not trying to buy the cheap/low power LED replacements as their intensity is insufficient. On a motorcycle, stop/tail/turns cannot be too bright. What is your life worth?