ideally you would remove the guides, get some new ones, turn them down in a lathe and then fit those.
It could be A significant improvement at higher valve lifts. Removing the guides may be a specialist job though, probably need to heat the head plus have a press.
The best reference I am aware of for the theory and practice of cylinder head modification is david vizard's book 'tuning your a series engine' although he does also have an earlier book called 'theory and practice of.....'
Even though it is about the a-series engine, it tells you what you would need to know to attack any kind of conventional tuning program, especially on a pushrod engine like the 126.
GAV