I threw this together quickly but hope it helps some. This does not cover every model made but the primary ones we discuss here most often. .
On the earlier models such as 102 or 122 all the way up thru the Quietlines such as 1200 or 1650 the first two numbers will tell you the horse power of that model. The 70, 71, 72 and 73 models had 7 HP motors.
I am not very knowledgeable about these early models and could be mistaken. If so we have some other guys here that can correct me. The following document should help with the other info:
982 comes with Onan gas motor. Forgot about that...