314-8

[bds1984 1,382]

That Cub engine will fit, bit it may not work with the manual PTO of the wheel horse as that engine may not have crank bearings to support the Wheel Horse PTO's lateral forces like the original Kohler does.  I went through the same issue with installing a Cub engine in my 520H years ago.  If you can utilize the electric clutch AND the drive pulley for the transmission, you are all set, but my engine only had enough crankshaft length for the drive pulley.  Not a big deal for me as I only use that 520 for plowing snow and pulling.  I don't know single cylinder Onans at all, but that PTO/bearing issue surfaces often when non-Wheel Horse spec engines are installed into a Wheel Horse.   

[Evguy 550]

1 hour ago, bds1984 said:

but that PTO/bearing issue surfaces often when non-Wheel Horse spec engines are installed into a Wheel Horse.   

I had considered this problem, I converted a gt1800 to electric and installed a thrust bearing behind the drive pulley and motor plate. So far so good, I may go with similar setup on this. 
I also need to deal with 1 inch shaft vs 1.125 and its shorter so its a challenge. I did get electric clutch but I would need to source a 1 inch pulley as well.

[bds1984 1,382]

I know the 16hp Briggs needed a thrust bearing installed but every 18hp I've had utilized ball bearings from the get go.  I'm glad you are aware of the intricitites, many are not and a perfectly good engine becomes a paperweight.