Several years ago I got interested in vintage Singer machines, and bought some off Ebay. There was one seller who demonstrated the strength of these machines by sewing several layers of leather belt, I mean at least an inch in thickness, so the stock Singer machines might be something to look at. There were models with a hand crank also, if you need that kind of precision, and if leverage is an issue you could attach a length of CR steel flat bar across the handwheel, and attach the original handle to that, something like 1/8 or 3/16X1" should be plenty stout enough for that.