What kind of Onan engine is this?

Hey all, just bought a OMC Mustang 320 skid steer, I want to do a complete service to the motor but can't figure out what Onan engine it is, I was told by the previous owner it's a 25hp Onan, I figure it's a B series engine since there's no oil filter and I have points ignition, but can't figure out which one.

I believe the engine was built: August of 1978

just in case you do not know this cummins diesel owns onan now

Thanks guys! I got the engine freshened up with a valve adjustment and de-carbonization, new gaskets all around of course as well, she runs great now! But I seem to have another problem, the drive belt shredded, not sure why, I thought I had the engine aligned properly with the hydrostatic pulleys but after driving it for awhile I noticed I had pieces of belt slung around, took a look at the belt and the original thick belt with 2 grooves had separated into essentially 2 belts. I'm trying to find a replacement but having a hard time figuring out what kind of belt to use because the original was so shredded and worn I couldn't get a part # off of it. Any idea where to get a belt?