hey jay! welcome to the [s]madness,[/s] i mean family!
with regards to the gas gage, i've never seen one, but i'm sure there is some aftermarket one some wear that could be fit. maybe a gas cap of some kind that dangles down internally? it really isn't that big a deal to not have one, as they get decent milage, always north of 65mpg, and as much as 100. many do add larger tanks of some size.
the seat can be shaved down to knock off an inch or so, though that compromises the already substandard cushioning. you could replace the foam with some gel material and be ok.
search this forum for lots of answers to questions you will have,i have found a real wealth of knowledge on here and friendly people willing to help.