Without better knowledge of the design I can't make specific reccos.
However: there are some pretty generic mods that apply to most sidevalve engines.
1. more compression.
2. piston to head .035" @ TDC.
3. sleeve the intake guides down and run a smaller stem valve (5/16"?) with a larger head, like 1-7/8" or more if the seat isn't too close to the bore or exhaust seat.
4. 30° intake seat (if not already).
5. the exhaust may already be big enough, but the valve should be re-contoured with a back-cut etc.
5. relieve the edge of the factory seat or relief facing the bore into a 1/4" radius (or whatever fits). Don't extend the relief all the way to the bore, just ramp it up to the deck.
6. I'd measure the inter-port centerline distance (between #1-2 and #3-4) and see if any other engines (incl. OHV) are really close. As a guess, it's between 7 and 8". It might be close to the Ply/Do 218 & 230, or Rambler 184" etc. pre-1965 L6 motors.
7. the obvious new manifold is 3 head flanges, 3 base flanges, and 3 tubes for a triple sidedraft using 3 × 1-1/2" SUs.
