50cc topping at 25, low compression?

[Noismute]

Hey everyone, Brand new to the forum here. Forgive me if what im posting here already has a thread on the forum, I've been trying to register for about a week and for some reason it JUST now let me through. Its Sunday, so i just wanna try to get an answer before this issue keeps me off the road any longer.

I've got a 50cc gy6, bought it new about a year ago and put about 10k miles on it going to and from work. It used to sit comfortably around 40, but now it usually tops around 25. I've replaced the carb, tuned the carb, Took the main jet to a #85, replaced everything outside of the motor and gearbox and there's still issues getting it kick-started(electric starter went out) and accelerating. Although i can feel a slight increase in power from the new carb and main jet, Theres really no increase in speed at all. I think it might be low compression, though I don't know how to test it.. So the question I wanted to ask; IF the problem is compression, Will properly installing a big bore kit fix that?
A new question was brought to mind while typing.. Does anyone think the engine could handle that kind of extra power? this is the kit i was looking at. Money is an issue, which is why i chose this specific kit.

http://www.ebay.com/itm/100cc-Big-Bo...56a5a7&vxp=mtr