assuming you have done all the usuals, bearings etc check and ensure that the crankshaft bearings are properly bedded. you might not believe it but one can get the alignment of the bearings skewed and even 1 degree misalignment can case the sort of problems you are talking about.
depending on the series bore size is a good start to check engine size. problem i find with these engines is that capacity is usually varied by the size of the con rod which leads to other interesting situations. the best one i had was an os 20 fourstroke that went like a bat out of hell, couldn't figure why but it was good in comps, (i had bought it second hand). when i pulled it down i found it had os26 fourstroke car bits inside.
use the factory part numbers when replacing bits and you should be ok.