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Brent
There are too many variables to your question, too thin a wall will cause other issues.
If memory serves, 1/16" of calcium/rust build up equates to multiple inches of cast iron in regards to thermal transfer to the cooling water. That said, I would think the condition of the cooling system and especially the water jacket of the block would need be cleaned in order to perform satisfactorily. I have done the acid soak in preparation of filling with aluminum, amazing what came out.
In a low performing motor less than 1/4 hp per cu in displacement the peak cylinder pressures will not be affected by the reduced sealing capacity between the 1/2 and 3/4 cylinders. .125 oversize reduces this web by 33% (from 3/8" to 1/4") if the bores are not moved away from the web. This is where the games of adding a wire to support the gasket comes into play, just another patch.
Another issue with going big on the bores is finding pistons and customs cost a ton, sleeving is less expensive if you do not need a forged piece.
I have seen blocks break just under the water jacket separating the top and bottom, it took a load of nitro but still broke all the same.
Best of luck, John