The overheat, if it was caught in decent time and didn't seize the pistons, may've contributed to blown head gasket
= loss compression
and loss o' power. New head
gaskets are available for these if U decide to rebuild. I had the same exact thing happen to my M20- the thermostat failed and she overheated. Was caught right away, but still caused some warpage and blew the gasket
. Had to do a top end overhaul
and found that it wasn't that bad. Now she runs like a top.
It's worth taking the head off at least, clean out the carbon and peeking inside: valves burnt? Springs okay? any cracks in head or block? Pistons tight? cylinders clean or scored? Those tiny "swirl" type combustion chambers clogged? If she still looks good, it's worth a rebuild. save $$$!