91' Cadillac Deville GM say's it's not worth fixing

How much would a cheap engine cost? I live in Lansing, MI

I got a tune up 3 weeks ago, when I got the car back I had multiple lights on svc engine, stop engine coolant temp, and I was still leaking oil pretty bad. So I took it to GM and had them check it out and they said that the other shop never replaced my oil cooling lines they just did a temporary band-aid on the hoses, and that my number 5 valve is missfiring and that it was only giving 100 psi on the compression test. GM said to drive it until it dies and that it's not worth replacing, I would really like to keep this car, Soo... I want to know what would be the best route for my car? Rebuild the engine or try and find a better engine to swap it out with? Also If I can find an engine I might have somebody that will put it in cheap, what would be a good price for an engine swap?

No I believe the guy that I might be taking it to has all the tools to do the whole job there, so I just need to find a cheap engine that I can swap out for mine. I'm going to check a local junk yard, do you guys recommend any places to buy an engine from maybe a rebuilt engine?

Yeah but I live in MI I don't know if they have very many 4.9L V8's lying around the junk yard that's in better condition then mine. $400 doesn't sound too bad but how much is the rest going cost like pistons, oil rings, machining, also bearings and gaskets, and possibly some sensors?

drive it till' it dies then huh?