Hi Erik Some time ago I bought via the internet, a CD copy of the Hepolite piston and ring data for British and continental motorcycles from about 1930 to 1955. If I want an ususual size ring or piston, I have a look through in case there is a more popular model where spares might be more likely to be available. There is no certainty you will find what you need, but there is a chance and pistons that are slightly oversize can be ground down.
In any case, an aluminium piston will have much bigger clearences than the iron one which would run with about 0.035mm running clearence.
As regards castings, many years ago I was involved in making a production milling machine of about 100 total hp and to give the best possible performance, we made the machine bed from grade 14 cast iron as this absorbs cut vibrations quite well. As the machine was a “One off” special, the pattern cost would have been very high, so we had a none reuseable pattern made from expanded polystyrene that stays in the mould and is burnt out by the molten iron. It makes a lot of smoke but it is cheap and simple and you could carve out the pattern in two halves and stick it together. You should check with a small friendly foundry first, but it is an idea. It worked for us! Best of luck Roger
