OK, but this isn't an electronics issue. If you specified a motor fit issue in the first place, we'd be farther along. 
One solution is to find the correct sized motor and all will be well. You can almost surely order one from MTH parts.
If the tach is on the rear motor, you could remove the flywheel from the front motor and cut down the shaft. You could also find a different thinner flywheel for the front motor and just cut the shaft down.
