I've been watching for a deal on a suitable replacement motor and finally found one. This new motor is a two speed axial piston motor like the stock one, but it has a larger displacement and has external adjustments for minimum and maximum displacement limits. At the factory settings it provides 10% increased starting torque (for accelerating from a stop, pulling a load, etc.), 41% increased high speed torque (for maintaining speed up hills, etc.), and 29% reduction in maximum speed. I've had it installed in my Brutus for two weeks now and I'm very happy with the results. I can now maintain 18 MPH in high range up a steep hill where it struggled to do 10 MPH with the stock motor. High range maximum speed is 23.5 MPH and low range is 10.6 MPH, with adjustment it can reach 29 high and 13 low with 14% increased torque over stock.
The motor is a Bondioli & Pavesi M4MV50-5-/31. Surplus Center has a few of them in stock for $220 which is an amazing deal, these normally cost several thousand. Unfortunately it's not a direct bolt-in swap. It did require extensive modification to the pump command lever for clearance, and about $100 in hoses and fittings.