i rode one of those before and it was super confortable to sit on. but the engines dont last too long, mainly the carbs stop working for no reason.
if i were you i wouldnt upgrade it too much and just ride it as is. the frames on those things arent made too well and the quality of steel isnt too good either. but if you do plan to upgrade i would get rid of the dually in the back and maybe reinforce the frame abit before putting in a 5hp engine in place of the dummy one.