Towing the machine, letting motors rotate without oil, can cause damage to the motors is they are not already damaged along with the pumps. If you need to tow don't go very far or very fast. To tow it, use 2 wires with alligator clips at the battery, unplug the brake solenoid and hot wire the solenoid with battery voltage. Then use bungie cords to tie the steering levers to full stroke in the direction you are moving.
Regarding your 1st post: The engine and pump unit are installed as 1 package, fastened to the frame on 4 mounts (1 at each corner). If you can get the pump drive pulley off while in the machine you can remove the pump and leave the engine in. On a 763 there is not alot of clearance from the end of the pulley to the frame if you need a puller to get the pulley off but may be worth a try before taking engine and pump unit out.