upgrading the M97/20 engine

[svandamme]

Who has been upgrading the actual engine? and what has been done to it?
Starting with a M97/20 block, obviously the jugs are fixed. But can be sleeved.


What are the common options or machining to do?

I hear the stock heads flow pretty good, but is it worth trying to find M97/21 heads?
Does anybody have the specs ? Valve sizes for 20 vs 21?

What else is possible?

Don't say just swap in an S engine, since I can't do that , annual tech inspection would be a problem, they check the engine code.