Hi, A D2 with a stuck engine could be the beginning of far more problems. I had a D2 with a bad pony and a stuck engine. The engine broke loose without too much fuss but I discovered that the Valves & Seats were rusted out and needed to be replaced and also that the Rings were stuck in the grooves. After several years of collecting parts I ended up selling the project and bought a different D2 in better shape. If the machine has been sitting outside I'd be cautious.
If the Engine is known to be stuck I would advise checking the Steering Clutches (tow the tractor and make sure they release when you pull the levers), Brakes and Main Clutch before going any farther. All this is repairable and parts are readily available but the cost adds up faster than the National Debt.
'46 Willys CJ2A Farm Jeep, '49 International KB-7, '31 Allis Chalmers U, Cushman Scooter(s)
Antique garden tractors & outboard motors
Other rusty old junk comes & goes without warning.
The 2 most useful tools to have in your shop are a Crystal Ball and a Magic Wand