Replacement short block?

[cfcjr]

I may have a chance later this week to pick up a 2003 MM with under 80K miles from a local dealer that will be taking it in trade for short money. The customer complained of "rapping" and was quoted 6500 by the dealer for replacement engine. I am guessing a spun bearing. My question is: What engine choices may I use for quick replacement from salvage yard? Mark VIII Dohc? I know the heads are different, but I could reuse the original. Cobra from what year to what? I am planning to run NA so I do not want the terminator with 8.5 compression ratio. Any suggestions would be welcomed as I have followed these cars for years, but I could not afford one before.
Thanks
Charlie

[FordNut]

Marauder of course is a direct fit, Mach1, Aviator, '99-'01 Cobra work with intake manifold swap (maybe a few other things you can get off the old engine). Mk8 and '96-'98 Cobra won't work unless you swap heads too.