In general, to make castings for metal, use casting sand, but it's more work than having a mount made from a single piece of aluminum billet. Plus, if I were going to give it a try, I'd remove all the cooling fins from the flywheel,the starter pawls shave as much metal from the front housing as possible but still hold the magneto coil. The shaft may or may not unthread, if it does, then it would be easier to replace it with a bolt and buy or make a prop shaft coupler for an already existing engine that could be centered up and drill the holes and bolt it to the flywheel. But after all this, it may be cheaper to just buy one of those king hobby units or skim the classifieds here for a good used zenoah or us engine. The old US 25 could be bought for real cheap sometimes.