Sorry to hear of the problem. I can relate my experience in case it helps you decide on your options
I replaced the missing head on my blue '87 300D turbo with a used #14 head. The car had been donated to charity who wouldn't accept it non running so the PO had a shop pull the head to see why it was running poorly and then they didn't want to proceed any further so it was sold with valid title for $450 to cover the mechanics lien.
The head I ordered was pressure checked OK and it cost me $700 with a warranty. There weren't any used heads of the later run available and I didn't want to pay $1400 for a new head (sans valves)
Newer heads may require step machining the openings for the precomb chambers so this is something to be aware of.
Finding a good running 603 engine is a hit or miss situation and you may have to pay big $ to have it shipped to you. I haven't been on the lookout for one for almost a year but the last 603 pull I saw was $2500. You are taking on someone's problems too, so if you buy a used engine you should have a new headgasket put on it and check the head for flatness while its off, these aluminum heads are not very forgiving if they are overheated and they tend to corrode if the coolant is not changed periodically.
A bunch of other things also should be included if you want to bring reliability up to a par with a new engine and that includes a new water pump, serp belt idler pulley and tensioner, vacuum pump (very important!), sensors including thermo sensors and oil pressure sender (they tend to fail because the 603 uses electrical sensors) and if the donor engine has much over 200K on it you would be better off if you replaced the timing chain and tensioner and guides. Rings too, if theres money in the cookie jar. Oil cooler lines, trans cooler hoses oh me lets see, motor mounts, filters, clean the intake manifold (I've never seen an old 603 that wasn't half plugged due to EGR). That is all the items I replaced because I had access to a shop and time on my hands.
Aw heck I could have just bought a new engine but I saved a bundle and have a great running engine now. I skinned a few knuckles but I learned a lot about the 603
A crate engine is probably $6K or more