The engines are available, Ebay is one source, or a wrecking yard local
to you.

But if you do not have an inclination to do the work, it may not be worth it.
Ask the mechanic for the price to remove and replace (R&R) the engine.
My local independant mechanic wanted about $1200 to $1600 just for the
labor. I guess long gone are the days of the $200 motor swap.

Further though, with the windshield problem and leaking water in the back,
you being in the North East and all (not in the sunny dry places),
maybe it would be best to put it to rest.
That interior must have cost you quite a bit.

Maybe you should put that engine money into another car.
I picked up my 83 300SD for $1525, put in $400 in needed parts
and it is now good.

A car with a bad interior but mechanically sound might come your way.
Then you could do some light duty restoration work and end up with
a very good car.

Just my 2 pennies worth on the subject....

__________________
80 300D 340K Owned 30 yrs
83 300SD 440K Owned 9 yrs - Daily Driver 150mi/day
02 Z71 Suburban 117,000
15 Toyota Prius 2600 miles
00 Harley Sportster 24k
09 Yamaha R6
03 Ninja 250