Bullet you must be looking at a Taylorcraft or old J3 Cub
The O-170 is basically the same as the A65-8 (no starter or jenny on rear case)
Parts are all still available from a number of sources in the US like Fresno Airmotive in CA. They also carry stock of the parts for that old Stromberg NAS-3 carb.
I would go with new cylinder kits which include all valves pistons etc as to overhaul the old ones normally opens a can of worms. The complete kit of 4 cyls costs around $3800 from Superior.
The bottom end is fairly straightforward and as long as the crank and cam is good you should have no major surprises.
I would also fit new mags (complete right hand rotation kits are available with sheilded harness and plugs from Unison, IIRC the part # is K4334) To mess with those old Eisemans or Bendix bricks will cause you headaches
There are a number of good shops here locally that I am sure would undertake such an overhaul but dont expect to get away with much under R100000
for a decent job
PS: here is a good website with lots of gen for those little Conti mills http://www.bowersflybaby.com/tech/fenton.htm