Sounds like you are planning to change from a 6 volt mechanical shift starter drive to a 12 volt solenoid shift starter drive.
They are fundamentally different.
If you swap out the starter motor, make sure the new starter drive has the same pinion gear as the old starter drive. If you don't, we will probably be answering questions about chewed up teeth on the flywheel and/or starter drive.
If you can somehow mount the mechanical linkage for the foot stomper starter on the bell housing, I suppose you could try mounting the old pushbutton contactor mounted on the top of the old starter motor to the top of the new starter motor.
If you do that, DON'T connect the battery cable to the switch any more. The positive battery cable would connect directly to the solenoid. The "new" switch on the starter motor would be the pushbutton, operated by the foot stomper, to complete the low amperage connection to the solenoid.
Just be sure you have good spring returns on the foot stomper linkage and the push button the linkage engages so that the mechanism disengages and doesn't mash the pushbutton when you don't want it to, otherwise when you hit a bump, you could get a surprise as the solenoid attempts to engage the starter motor at speed!
