1986 fleetwood engine compatibility

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.

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