Cyric 30

Hello All.
I have a 1981 Toyota with a factory diesel engine.
The engine is a Toyota L type 2.2L.
The transmission is a L52 5 speed Manual.
ive had this truck since i was a teenager back in the 80s and have driven it repeatedly over the years, but the old engine finally started using oil, and alot of blow-by, way back when, and this was after a rebuild on the engine once already. ive been feeling nostalgic and wanting to get it going again, but the engines done for
things i loved about this engine, the gas mileage i swear 40 miles to gallon on flat ground,
things i hated, top speed of 60 miles an hour all out and hills.

What i would like to do is swap out the engine with another diesel ,but with something that retained the gas mileage, but has more HP to get up and down hills better, i know nothings probably going to get 40 MPG, but i can dream.

i would like this to be a fairly easy swap if possible, i don't know if there's any Toyotas that are a direct swap? or kits?
if possible, keep any electronics to a minimum, or if it does have electronics something that can be done simply, maybe a small turbo diesel of some sort?

There are a lot of power units around here as well so engines that might not come in a vehicle originally might be an option, lots of Kubota, yanmar, perkins, detroit, yanmar power units throwing around here.
what are you folks thoughts?

I thank you for your time and attention.

Cyric