Well first don't borrow trouble, it might not be anything major, you could keep a lookout for a 2nd user motor on evilbay or some such. But if it is what you think it might be then only buy what you need not what you would like, sounds trite I know but sometimes folks think they have to buy new or a complete set of something when really all they need is a single part, if your bike was for racing for instance you might need matched pistons but as it's not, a single piston from any source will do for now till your a bit more flush, say 30 years from now when your kids have left home. my youngest is still here and he's 23 FFS! Build it on the cheap, get parts when you see them hunt round local boot sales if you have to. Where are you in the world? someone might be nearby and willing to help out with some advice or even wield some spanners. Best of luck mate.