Cadillac: 1986 fleetwood engine compatibility

Hi,

I was lucky enough to receive a 1986 Cadillac Fleetwood. All the
exterior and interior are in remarkable condition, but the engine is
going to die any day. I fell in love with this car and I'm not ready
to send it to its grave in the junkyard. I would like to spend my
summer break installing a new engine in this baby.

Can someone please tell me what engines would fit into my car?
I would also like to know the price range of these engines.

Thank you.