I am looking at options to re-power a skid steer (Owatonna Mustang 330).  Looking at Honda GX630 and GX690.  When I review the specs at the Honda site, every measurement including displacement and torque is the same, yet the GX690 is stated as providing 22.1 HP @3600 rpm and the GX630 20.8 HP @3600 rpm.   Can anyone explain this magic show.  One re-power kit includes the GX690 claiming 26 HP. Where they got that number I have no idea, but they seem to believe that engine will work. The original engine for this machine was a 25HP Onan gas engine but I don't know at what rpm. I am wondering if I could go as low as 20 HP and still be fine or rather stay on the higher end?  Any comment will be appreciated.

3,600 RPMs are about the standard for engines as such, yours included..
 
I don't know the specific difference (very possibly a larger carberator) between the the 630 and 690 but I'd go with the largest, closest engine to what you had.   Better to be "over powered" than under powered.. Even with that said, the Honda is still smaller than the Onan.
 
BTW, the 690 is way more popular for some reason, you never hear about the 630..
 
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Thanks SRTsFZ6. 
Seems like I may start researching Kawasaki engines.  The FH72 looks interesting. It is 25HP and has a stub shaft on the flywheel side as well.  Any idea what the application is for this?  My thought is that it could be coupled to small pump to provide an auxiliary hyd. circuit.  Currently the machine has a military surplus Onan 4 cylinder, 24 volt start.  It is super loud and smokes a lot until it warms up.  It is a 1800 rpm engine running up in 2400 range for enough power.  Since the original gas Onan was likely a 3600 rpm motor the diesel would seem like a poor match to begin with.