Keep Sinking Money or New Engine?

[X5DC]

I have an 2005, 4.4 with about 105K miles on it. We purchased for around $10K knowing there would be some work to be done. The truck has great service records, mostly done at BMW. We purchased about a year ago and have done timing valve covers, oil pan gasket and timing chain.

The current problem is that there is a slow leaking that is hitting one of the spark plugs. I can't remember which seal it is, but the mechanic said it would probably cost $5-6K due to labor.

Would it make more sense to have the work completed in light of the repairs we have already made, or should I purchase a remanufactured 4.4 engine? I have seen a couple on EBay for $3.5-4K including shipping and come with all the needed gaskets, thermostat and water pump.

What would it cost to swap the engines? Would a new engine be too much power for an older transmission?

Thanks in advance!