The models with a H50-H60-H70 engines on them typically have a 1" x 2-3/8" shaft on them. The Briggs Intek OHV 5-7.75hp engines will normally replace these engines.
Additional Info
If you buy an engine with a 3/4" shaft, you will have to change your pulleys or purchase the sleeve kit to take the diameter from 3/4" to 1". You will need to replace the mounting studs with normal bolts for mounting. If your chute control rod goes above the engine, you will have to buy the same series of engine or modify it so the crank is out of the way.