You say you have another engine?? This is what
I would do.
Remove the oil filter from the engine that is presently in the car and carefully cut it open. Look at the filter element for signs of metal. White metal, as long as there is not much, would probably be from the head where the mismatched caps have worn. Any ferrous metal, which is attracted by a magnet means that something may have gone through the the oil pump.
If all is well, then remove the head from your spare engine and give it a bit of a service, ie, lap the valves in, fit new valve guide seals and give it a decoke. If funds can stretch that far, give the face of the head a light skim, purely to reface it and take out any deformations rather than to give it any performance gains.
Then fit to existing engine and reap the benefits of your labours. If you need a helping hand, P.M me. But there is no need to put the car into a garage and hand over money when help is at hand.
This need only cost you a head gasket set, and some time. If you can, a new set of followers and a reskim would be beneficial to round off a decent job.
