RE: what engine you invest you money?
You are going to get lots of answers here to this question - depends what you are going to do with it.
For stock, I have recently been very impressed with a Sikk motor that my friend bought and we chucked into his Panther. It runs great, and, for the price, had rubber mounts, and a nice throttle linkage that made radio install a breeze.
I run a tuned Zenoah and for higher performance I have so far been very happy with it. Lots of tuners out there and most people have their favourites.
There are lots of petrol engines I have not run either, so you may find some other answers. (I hear that CMB are making a 25cc motor this year too by the way).
So, my choice for stock is Sikk and tuned is Zenoah.
Glenn