Many forklift motors have a tail-shaft which goes to a brake drum. Because the drive-shaft goes directly to the differential, locking that brake parks the forklift.
On some motors, the driveshaft and tailshaft are the same diameter and spline count (but on some they aren't!
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You can remove the drum brake, get down to just the splined center, and move it over to the DRIVESHAFT end. It can then become the basis of a coupler to the transmission shaft or flywheel or flywheel replacement.
Beats trying to dig up the right size splined connection somewhere else!
PS - On small front-wheel drive cars, your motor limiting factor is the diameter of the motor (distance from center of transmission to half shaft on the motor side) and the length of the back of the motor to the wheel/tire.
On my electric Geo Metro, the 10.5" diameter just cleared the half-shaft, but the tailshaft was too long. It would have popped the right tire on a left-hand turn if I left the tailshaft on. With it removed, everything was great.
Your motor looks good. Sounds like you are off to a great start!