You've got to remember leaks around the rings are still possible. Generally twenty percent or greater losses is when you wanna fix something, but as little as 10 is enough for some folks. You might hear hissing in the exhaust if it's the exhaust valve, intake hissing if it's intake, obviously. You might get lucky and even get hissing out of the crankcase somewhere if you were to have bad rings. Not all the time, but sometimes. Get your valves right before testing. If valves are in spec and you're still getting a poor reading, try a teaspoon of oil in the cylinder. If improvement is dramatic, consider replacing rings. If you open the oil filler cap and the reading doesn't change at all, you have the best sealing piston rings on earth.
Truth is, with it being a simple, relatively low compression engine, a compression test should be enough. Pretty similar ideas, only you'll be working with PSI's instead of percentages, and you're blowing out instead of in. It won't be as accurate, no, but it should give you a heads up on any issues. If it's running well and you have compression within spec, I wouldn't worry beyond that. Leakdowns are more helpful when there's a cooling system and a more complex head gasket to leak from, PCV valves, turbos, and the like. Still yet, if you can't get good compression and are hesitant to replace rings, then maybe try the leakdown test. There would be a slim chance it's your head gasket, and that's the test to let you know. If you have both testers just sitting around, sure, use the leakdown. It's not a massive difference on a TW.
That's about all I know. Somebody with more than a shadetree degree should be along shortly to help.
EDIT: I forgot to mention. Kinda obvious but you'll want to be at TDC on the compression stroke for a leakdown.