The result of that thread is a carburettor swap, some tinkering with the jetting and realising the motor has an oil control issue. It seems like, or at least I hope it seems like a valve stem seal issue.
What I would like to know is there a reason why I can't do it with the engine assembled? valve train excepted of course. It doesn't live in a Subie right now and making space or even removal isn't massively hard if clearance ends up being an issue.
So, can I do it? What's an adequate type of spring compressor for the job? Where can I get seals at a decent price?
Yes, I am trying to do this on the cheap. That's why I don't want to remove the heads. If it's not the stem seals then the motor is essentially cactus anyway, so why bother getting nice new gaskets?
For fun, this is what I'm dealing with:

It's been cleaned up more since then, but that's more or less it.