What motor will fit my 2004 Honda shadow spirit 750?

All the RC44E engines made between 2001 and 2007 will fit straight in. Any other Shadow engine will require some sort of modification, if you wanted to use it.

Get the same engine again from a wrecker... fortunately for you, cruiser bikes are not at the cutting edge of performance (and don't need to be) so manufacturers tend to leave the engines alone for many years, except for minor upgrades like fitting fuel injection from carbies for emissions reasons. So you should be able to find one that drops straight in. Also, depending how "blown" your old motor is, don't discount getting it rebuilt. They're not super complex motors.

Almost certainly, but to be sure, I would join an owner's club and ask their advice.

A Honda Shadow 750 of course.

Stick with the Honda 750. Honda made, and is still making, that same bike and engine for years so there should be many used engines for sale. Just be sure the output matches what you have, shaft or chain. A used one should come with the transmission gears, clutch, stator, covers, etc. Possibly not the carbs, however.

Several years ago there was a guy trying to put a VT1100 engine into his 800 Pacific Coast. Consensus was that it would work but he didn't come back to the forum to confirm. The 800 would have been the same basic engine as yours.

I had a '95 VT1100 Shadow for years. Still miss it.

You could probably make a couple of different years fit - but I would try to find one that is the same year to keep from running into problems with it mating up to the transmission, motor mount locations, wiring harness, etc.