You can buy a new cylinder head for a 1981 VW Rabbit Diesel at most auto part stores. Salvage yards also carry the item.
I've had my Rabbit PU for 11 years. I always use 15W 40 oil made for Diesel engines.
The firing order on a rabbit is 1-3-4-2 gas or diesel.
1000 miles
50% regular ol' anti-freeze (Peak, Prestone, whatever) with 50% water.
There is no 1991 VW Rabbit.
Coolant for a 2006 VW Bettle is put into the coolant overflow tank. As it is needed, the radiator will automatically pull in additional coolant.
A 1881 VW Rabbit convertible would be worth its weight in gold; however, I believe you are referring to a 1981 VW Rabbit convertible which is worth far less. In *perfect* condition, with the right buyer, and low miles, in a good economy, and diesel its worth about $3k. In good condition (diesel) its probably worth $1750. In great condition (gasoline) it should be worth $1000. In fair condition $500.
I'm afraid won't be much. Check Kelly bluebook site & run the numbers. That will tell you & is what the dealers use as a guide.
The recommended spark plug gap for a 1981 VW Rabbit is .028"
VW will tell you that you must use their specific yellow coolant. You don't. Just use the top of the line Prestone that is for all engine types. Don't add water!! It runs on pure coolant...
Recommended coolant/ antifreeze for vw, audi, skoda, seat is G12. Very good coolant, helps, lubricates an works as a anti-corrosive serum.