First off, welcome to OCC!
That's going to be an undertaking.....
Does your dad still have all the parts he removed? That would make it a lot easier.
Your going to have to get a gearbox. You can't "fab" that up. I really think this project is far beyond just needing measurements. You'd be better off buying another tiller, and copying it. Or seeing if there is someone near you who would let you take pics and stuff of theirs.