This is what you will need to convert a STS 21B to roto start.
#280105 Roto-Start Backplate Conversion Set $16.99
#280103-4 Pull-Start Adapter w/one way bearing $12.99
#280103-3 Pull-Start Backplate & O-Ring $11.99
#280200 D28M SG Crankshaft $49.99
Total $91.96
Prices will probably vary but this gives you a good idea of what it will cost.