Many B's and C's have been equipped with a 5' Woods belly mower and done just fine. Be warned, you're looking about 4 1/2 hours to mow the lawn! I bought a 6' finish mower and mad a rig to drag it around the yard - but it was too big for the B if the clover was tall.
These are a bit smaller HP than the ford, but what they trade off is weight. They go 2000 lbs versus the Ford's 3400. 9" turfs and a set of GM or Ford 15" wheels (mine were Camero spokes) with low pressure up fron and you'll not leave a mark on your grass.
Problems? Check for oil running out of the brake ports at the bottom, indicating a bad seal. Check the final dives for slop - with the ighnition off, tractor in 1st gear, see how much you can move the rear wheel before it comes up agains th motor. THis also will show up as a "slip-bang" or general slop when releasing the clutch, particularly going from forward to revers. It's pretty easy to adjust a bit, but major slop meens new bearings. ALso, the older tractors often need valve jobs - watch for surging or dying under load when hot, or excessive smoke. Check for excessive slop in the steering - again, adjustable to a point. Check to see if the drawbar mount hole in the final drives are not broken, and that the clamps that holed the drawbar can be removed intact. Starters that skip indicate a chipped ring gear on the flywheel.
Guess what - that's about all the parts there are!
Watch your water temp - dandelion fluff will plug the radiator!
Mags are more forgiving of living outside but the distributor actually performs a little better. A good tune-up is a must, if run lean they'll burn the valves.
Watch the PTO - it is not live or independant - when you push in the clutch the mower will drive the tractor via the fianl drive. A $40 overruning unit will fix this.
Also, the early PTO shafts are undersize, you need an adaptor to run most equipment.
Check out the posts for the guy who put a loader on his - Way Cool although I hope he as a spare front end
PS I love my little Allis B - It's getting a $1000 makeover right now!