I had very similar symptoms, I changed head gaskets, thermostats, etc.
At one point, the car was running at around 8mpg, whilst cruising, just because it was trying to keep itself running and not die.I finally decided to bite the bullet and rip it out and found that one cylinder hadn't fired enough to make even a slight sooty deposit. I haven't even bothered checking the liner, it's goosed.
I have looked at a few options but Turner offer a 3 year warranty on their engine, you can go for just the block, all the way up to a fully rebuilt engine unit, depending on your budget. I'm probably going to go for just the block, I have the rave manual and space to do the work so I will just rebuild it myself.