a couple of questions-is your block cast iron or aluminum? Whats the stroke of the crank stock and of the one you want to use? Bore size now? Valve size and stem diameter?

You won't pick up much in cubic inches on small bore engines, even less if they are 4 or 6 cylinder

as for rpms, size of crank bearings make big differences-we are using Honda sized ( less than 2.0 vs over 2 inch rods and now are using 9 roller bearings on camshafts vs old 5 bushings, valve spring pressure is so high that the cams want to break------also valve lifts are near an inch where as stockers around 1/2 inch.

Problem of big bore size weakens block and its hard to save deck strength for head attachment, many blocks studs will go to bottom main area and attach load to same area as crankshaft bore.

Measure your block length for cylinder bore spacing-----maybe theres a head out there that can be adapted ---I'm thinking something maybe like the Nascar RO7 stuff