I have a MTD yardman log splitter 21 ton , 5hp B&S model number 12H702 type 1772-D1

a mechanic told me that the problem is a flap on the crankshaft that opens the exauhst valve for compression relief , the problem is this is opening while running and killing the engine. Can I use a vertical shaft engine that has the right diameter shaft and shaft length ?

Thanks Paddler