Your options are to short block your existing engine, or get the crankshaft diameter and length from your old engine and then match it to whatever engine you are interested in. the mounting pattern will be the same no matter what brand of engine you decide on. the other thing is making sure the replacement engine has a large enough charging system. I think your old engine maybe a 15 amp system, but anything over 10 amp would probably be fine. Would require a different exhaust system, and doing changes to the wiring to make it work.
May want to look into the Kohler 5400 series engines which would most likely be a easier replacement than going with another brand of engine.
