Get another M42, they're not expensive. Put that one on an engine stand and start whatever project you wanted to do and on the plus side, you'll have all the time in the world and still have a driveable car.
This is a possibility but only as a last resort at this point since garage space is at a premium right now.
As for starting ANOTHER project, haha, I wish you can see my running list but here's a short and partial version:
1. Finish regut and rehab of entire second floor by this spring
2. Finish ITB's and COP for M10 motor
3. Install MSII on Tii
4. F/R BBK, bearings, rear LSD diff, full suspension, rust resto on Tii
5. Swap motor on Aprilia RSV
6. R&R Push Rod seals on BMW R75
7. Install 2nd floor loft in the garage
8. Drywall/Insulate the garage
9. Cut down big tree in front yard
10. On and on...
As for spark plugs, they are new and don't see how it can cause it to rattle. Hoping to finish dissassembly tonight after work.