If you can get ahold of "Top End", a book published by Light Plane Maintenance, the book has a good discussion of whether to top an engine or overhaul it. If your's is a high-time engine since a (real) major overhaul--more than say 1500-1700 hours--I wouldn't just top it, I'd do a major overhaul. They are not hard to disassemble, send all the parts out for inspection/overhaul, and (with some A&P help), reassemble.
However, O-235 parts are said to be a fair amount more expensive than O-320 parts because there were far more O-320s made. If you decide to do a major overhaul, I would suggest trading the engine in for an O-320.