Actualy, the Graupner Speed 900's are Johnson 51mm diameter motors.
Several variants are available trough Ebay.
Also, as long there isn't a disk or axe slicing trough the motor, for drive those are rock solid.

Afters 9 years of hard use , I know of 2 failures of a speed 900 or equivalent motors in a drive application.
1) Hannibalito 1, because Little Hitter dented the body and broke the magnets.
2) Plofbox II after a meeting with Hornet II. The axle was bent 90°.