You are correct in you reasoning, in that some conversions have more "potential" than others. I have a 46cc Poulan that is converted, and its a very strong engine. I think any of the chainsaw engines, especially the ones with Dolmar cylinders, will be the best to convert. An Echo #CS510 would be the best (same as a BME-50), if you could find one. I would also reccomend using a C-H syncrospark for the ignition. They are easy to set up, and really make the big gassers easy to start.
Z