RE: What motor is this
well - without knowing the normal performance of those engines, the biggest question would be the displacement. For example - the Brison 2.4 would fly a 16# plane aerobatically if you managed your momentum properly - it wouldn't provide unlimited vertical, but it wouldn't be a complete dog. The Brison 3.2 would be better suited for that weight range.
If that engine you're looking at is the 3.2 - then you might want to find out what carb is on it - and compare model#'s to something like the other 50cc engines on the market. Might be that you could move to a larger bore carb and et a little more power out of it. Plus if the prev owner had too big or too small of a prop on it, that could have hurt vertical performance as well.
What kind of $$$ are you going to be able to pick it up for - that might help us determine if you're getting a deal or not.