Lane,
I have one in the lube trailer, the military rates it at 6HP at 3600 RPM, but I've seen results in a google search of up toward 20 HP before (since the military under rates the HP output of it's engines).
Keep in mind though, because of the torque characteristics of an electric motor, you probably will not need as large of a motor as you have in the engine...if you replace a 10HP electric motor, you'll want to get at least a 15HP engine because as the load increases an electric motor will create more torque the slower it goes whereas a gasoline engine will only produce torque till you slow the engine down below the maximum torque RPM rating, then the engine will begin to slow way down and stall.
So it's going to be the reverse for an engine to an electric motor, but with the military ratings of their engines, if you got a 6HP electric motor 3600 RPM, you'll probably be good.