We have several parts tractors, haven't had to replace much on the gear case. We had to make new clutch pads, and small items like nuts and bolts. Engine wise unless you are a restoration person, go with a predator 6.5hp from harbor freight. We just switch back to one from a 3hp because it was always hard to start. My son is working on a david bradley with a custom lifan engine to fit under the hood. Most of the later version did away with the hood. Looks cool more complicated than needed.
Alot of parts will have to be fabricated or used parts found, hardest parts are the clutch assembly. Looks like everything switched to belt tension, these use a fly wheel and friction plate assembly. There are 2 friction pads inside, belt attaches to an idler and the clutch fork pushes on pressure plate clamping the idler to the flywheel there is a friction pad on both sides of the idler. There are oil holes for the bearings, but no oil should get on the friction pads and dont over oil them. These units need constant oiling and greaseing, we have a oil can and grease gun with the machine at all times.
You have to love old equipment to care for these units but they last generations. The engine bolt pattern fitts 6.5hp or briggs 5hp as is, we modifyed to fit the 3hp. Contact me if there is something you're looking for, we can also help with con version to other units, we have had problems with the clutch and are looking to swap one of ours to a friction belt or a modern clutch, but as we stole a clutch from the sickle bar we haven't done it yet.
Thanks
Brian
3H.R.