Lets get back to basics here. What do we need for the engine to run?
1. COMPRESSION
2. CORRECT AIR/FUEL RATIO
3. SPARK
4. Strong enough battery to crank motor (My addition)
Your rear engine didn't "suddenly lose compression" during the test flight. Your engine didn't take the vax, did it?
So lets rule out compression.
You could have an issue with the air/fuel ratio.
You could have a magneto issue.
If the rear engine won't swing the prop to start, you either have:
A. A weak battery - VERY common - Doesn't explain loss of power in flight though
B. A weak starter - Doesn't explain loss of power in flight though
C. Hydraulic cylinder lock - One or more cylinders are filled with either oil or fuel such that it can't be compressed and the prop won't swing. COULD possibly relate to the loss of power in flight. You reported the engine dripping fuel, so I'd look closely at this one.
I would charge the battery and pull the lower plug on all cylinders. Make sure the prop moves freely with the plugs removed. Does any cylinder have oil or fuel leaking out when you remove the plug? There's your culprit.
No liquid? Now check your mag timing - Mechanics are notorious for not tightening the screws properly on the rear mags due to inaccessibility. Loose mags slowly coming out of time would exhibit exactly the type of behaviors you describe. If firing too early, could also prevent engine from turning over (maybe).
Mags OK? Now check your mixture control.
Do all this and report back. If you don't solve your issue, I will continue to provide assistance.
Best wishes on solving your issue quickly.
