Check Agri -supply- they have a 40hp gear box that may fit- cost is 99.00. the bolt hole pattern is listed and it also lists as a King Kutter replacement. It has 2 different models, one with a six spline input shaft or a smooth shaft with shear pin. The output shaft is 12 spine tapered.
I just picked up a older model Woods rotary cutter that needs a gear box as well. I plan on purchasing this gear box and modifying the deck to accept the bolt pattern.