Don't overlook weight.
These things were designed to run a rototiller all day at full output so they're indestructible, but a 20 hp Yanmar does not have near the weight, and therefor the traction, of your Ford. I think the Yanmar match for it might be up around 26 pto hp plus some ballast if you need to pull a plow, etc.
Comparable tire sizes (2wd) might be a first clue to comparability.
If you just need 20hp to drive the pto, then 20hp is 20hp.