Wow!
Did you get the starter fixed? An airsoft gearbox motor might suit your needs a bit more than the modified motorbike starter.
They run from 7-12 volts. Generally from a lipo or Nimh pack. And can turn from 10K rpm high torque, to 45K rmp low torque. High Torque models are often really aggressive neo magnet based and hard to hand crank, speed models are normally ferrite and easy to hand crank.
The pinions are remove-able and are either grub screw on flat, or thread lock on round. So you could make a starter dog for the rear or front of the engine. Leaving it engaged would not damage the motor either as they have bearings and are mostly balanced. You can strip them (pinion off, 2 rear tabs opened, slide out the rotor to the rear). Then drill balance the rotor and re-shim if needed.
A man of your talents could make a lovely pistol starter with the batteries inside.
Normally they run anywhere from £10-60 depending on the brand (brand is everything in airsoft). And brand often means very little, It's a name and they probably shimmed and balanced it for £50 more.
http://shop.ehobbyasia.com/upgrade-part/parts-upgrade-aeg/motors-parts/a-p-s-ev-edge-motor-with-bearing.html#.WPFjamehfZI