You should call your local "power transmission products distributors". In my area they are companies like Motion Industries, Kaman Industrial, others. Ask them. What they will do is call the gearmotor manufacturers like SEW-Eurodrive, Dodge, Falk, Nord, etc., and see if a cross-over is available.
Most likely you will NOT get an exact replacement, but will require drilling new mounting holes or fabricating a simple adapter plate.
Before you call, get all of your data from the reducer's nameplate: input horsepower, voltage, shaft size, ratio, output speed, etc. Keyway configuration, mounting position, etc. would be useful to know, also. Gearmotor manufacturers will usually do a custom fab for a price if necessary.
TygerDawg