Clean surfaces with solvent/thinner and dishwasher powder.
To smoothen the piston head I use a Dremel and its accessories.
To clean the valves - I use a rotating steel-brush on my stationary drill.
(Sorry I do not know the correct English words for these tools).
Install valvestem caps when you reinstall the heads on the bike.
The caps are VW racing parts;
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It is a really small cost.
I've been using caps for the last 50 000km or so. Easier to adjust the play. More stabile play. Better lubrication; 2 vs 1 surfaces. Saves the valvestem butts and the rockerarm.
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You should aim at smoothing all edges to be as lean/smooth "as a babys bottom".
There is a 42mm highflow valve that you could install instead of the Y-shaped original BMW/TRW valve:
Bugpack 42x112x8; it needs to be shortened and new collet groves made.
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Art.no.: 4045-031
Price: 235,00 Sek (=½ price compared with BMW/TRW)
The highflow, T-shape, enhances the volumetric degree, and makes the low-revs stronger by about 1 gearshift at 2000rpm.
Check the guides - they may need to be renewed...
Check the piston rodbearings.
Check the stubs - they have a tendency to get loose causing airleakage.
New pistonrings perhaps?
New cylinderbase O-rings and pushrod tube rubber seals.
New headgasket.