Plenty of expensive work here to fix this problem, handy work of a previous owner. As a new R100GS head plus valves is £600 and secondhand ones are like hens teeth this is certainly worth repairing! Old bikes are never cheap until they are properly sorted!!
I’d be interested to see the repair before you put it back together. Bob
Cray Engineering in Kent repair these. They did myR.50 heads.
Contact me if you want their phone number, or they advertise in the Journal.
John Wheeler