gen1 :
valves made of usual "valve-steel" .
their valve plates can/may be regrinded
but be careful - if you grind too much, and also reground the valve seat, it can ! happen
that the shims have to be a lot thinner than the ones previously installed.
if you then get into the range of shim sizes of 245 or even smaller, you should shorten the valve stem end(s) by 1-2/10.
gen2 :
NOT & never and , as far as i understood , forbidden by suzuki - valves are made of titanium - only their seats can be grinded.