norbe,
You left out a lot of information in your question. 50 to 65 cc is a big areana and there are lots of engines that fall into that category. On the other hand you stated that you wanted the best power to weight ratio and don't want to spend a lot of money to get it. You're not going to get the best power to weight ratio if you don't spend the money to get it. It's that simple!

Put the names of all the engine manufacturers you know of on a board and throw a dart at the board. Keep throwing until you hit an engine you are willing to pay for.

There are lot's of engines out there worth having but if you want the best in the size range you stated then you better be ready to spend at least $600.00