sorry for the delay ok

yellow bit - leave the valve guides full length but machine down the end on an angle/taper so it's narrower toward the tip, but don't go overboard - long guide length supports the valve better and gives longest life. Cutting the guide shorter so it doesn't stick into the port at all usually gives some cfm, but the cost in valve/guide/seat life isn't worth the tradeoff imho. Not ever.

Blue bit - if you wanted to make the port bigger at all - then that is where to do it, leave the floor as much as possible, it's the loweest flow area. raising the port (if it has to go bigger at all) alters it so it is (even just 1-2 degrees) more downdraught, which is usually a safe bet, at least for something like this The red bits - you can take away most of the valve guide 'boss' (the extra area supporting the hole the valve guide is installed into) in the roof, as long as you don't go overboard. that gets rid of a noticeable flow obstructon.).

On the floor, on the short turn radius, don't go ridiculous, just make a smooth blended curve there. don't take it as far as the diagram - basically the coloured bits are hte bits to remove in general. The short turn radius was difficult to try and draw the curve using mspaint - so go by my description, just curve it off a little, remove the sharpness to the turn. Don't lose too much sleep beyond that.

This isn't about the worlds most highly developed race port, it's about what you could do at home and make some reasonable improvements - they are obvious stuff based ont eh cross section pics (I don't remember who uploaded the originals, but I thnk they were h89?)

I'd say at a guess you'd be looking at maybe 5-10bhp gain over a stock head on a combo using extractors, perhaps twin SUs or a bigger single carb and a moderate streetable cam (but something bigger than stock)