The hydraulic system in these tractors is certainly capable of powering a towed mower deck with no problems. The issue is one of finding a suitable hydraulic motor and then mounting it.
Mower decks are designed to spin at a certain RPM so that the blade tip speed is around 18,000 feet per minute. For this reason, the driven pulley that sits on the top of the center blade spindle on a 44" deck is smaller in diameter than the driven pulley on a 48" deck. The blades on a 44" deck are shorter in overall length than the two long ones on the 48" deck and the engine must spin those blades a bit faster to get the blade tip speed up to the 18,000 FPM mark.
So, in order to figure out which hydraulic motor you need, you first have to figure out how fast that motor has to spin. Choose the deck you intend to modify and then measure the driven pulley to get the diameter. Then measure the driving pulley that is part of the PTO clutch.
Assuming a WOT (wide open throttle) speed of 3600 RPM, you can use the math to determine what the original blade spindle speed was. If the pulleys are identical in size, then the spindle speed is 3600 RPM max. If the driven pully is smaller by a certain percentage, then the spindle speed is increased by that percentage. Conversely, if the driven pulley is larger by a certain percentage, then the spindle speed is likewise reduced.
The motor you choose should be a geroller type because these are high torque motors and very forgiving of oil contaminants. Most likely, your tractor's pump was designed to put out around 8.5 to 9.5 US gallons per minute at 3600 RPM. It will likely have an internal displacement of about .62 cubic inches per revolution. You should try and find a geroller motor that is no larger than this displacement so that it will spin at 3600 rpm when the engine is at WOT. This would allow you to move the center spindle of the deck to the right side and the right side spindle to the center.
You could then have a machine shop cut a longer threaded section on that spindle so that the lock nut could tighten directly against the lower belt pulley. Then have them reduce the upper part of the spindle to the same diameter as the output shaft of the motor so you can use a small LoveJoy coupler to join the motor directly to the spindle. You would also need a pump mount that would accept the motor on one end and would allow you to bolt the opposite end to a new mounting plate of 1/4" thick steel that would go over the lower deck pulley and be thru-bolted to the deck housing on three sides. Of course, this plate would have to have a hole bored in it to allow the spindle shaft to come through and perhaps the length of the spindle may have to be reduced to make it all work.
If the stock spindle proves to be too short, then any competent machine shop would have no problem whipping up one that was correct in all aspects.
The alternative to the above would be to use a geroller motor with a displacement of about 1.2 cubic inches with a long output shaft so you could put a large cogged pulley on it and use a self aligning bearing to support the end of the shaft. You would then use a cogged pulley that was half the diameter of the pulley on the motor and place it on the center spindle. This oversized geroller motor would spin at half the speed of the other motor and the belt drive would compensate for the slower turning motor.