Depends on what's wrong with it. These engines are basic, 1950's/1960's cast iron pushrod technology, and of a sufficient quality that unless severely abused wear tends to be withing quite reasonable limits.

You don't describe why it needs rebuilding. Assuming the basic "tired engine" problem, pull it apart, inspect the bores, if they're still good (you might even find original hone marks) change the rings, maybe new bearings, skim the head to ensure a perfectly flat and clean mating surface, you might have to change the valve seats (these were often not hardened, and may have worn away with the use of unleaded gas) and guides (have a machine shop do both of these), relap the valves and reassemble the unit.

If there's a specific problem of course it needs to be fixed, but again this is engine tech as basic as it comes.

Bram