M52 engine, plain and simples
The m50 hasn't got a stitch on the m52, I should know I've had both in my car one after the other!
But on a serious note, non vanos chips are easier to come by than vanos friendly ones I found when I had mine.
Set of cams, chip and a decent flowin exhaust should see good gains. Don't forget to add an m20 flywheel-clutch combo if it isn't already fitted, this will liven her up a bit.....
I'm told the internals on the iron blocked anchor will take around 350-400bhp without needing uprating (talk about being robust lol), anything up to 600bhp is achievable but would need a fair bit of coin and stregthening to run safely
Ant