On our racingcars we use starter motors from Quarter Master. low reving no small gearbox with plastic gears, but a very strong permanent magnet.
Proper roller bearings both side, aprox 3 Kg
The good thing is that these starters only pop out the sprocet and not like the usual way that the sprocket slides in and out over an axle kept in a half open alu case.
With other words you only have to drill ahole in the engine plate as big as the sproket, just below the distributor.
Not having this half open case around the sprocket is posible becouse the bearings are very rigid and the shaft is strong being about 19mm in diameter
We bought them from Super Clutch also run there 7" cluches, perfect fit, better than Tilton. Friendly guy Tony Tewson.
can't answer mor now, we are of to Londen for the coming week.
Hapy Holydays to all.