My take on things,some may disagree.
If the bike has lasted on the standard seats etc for this long and is not in need of any other engine work,leave things alone.
I guess the bike won’t be doing huge mileages,so just keep a check on valve clearances.
No harm in using additives though.
Agree with the above but I'd make sure not to put any E10 in it.
So called "preservation grade" 97/98 E5 would be my choice.
As it is for all my bikes.
I'd only use additive about every fourth tank of super unleaded but TBH the valve seats will be well hardened by now and an additive probably isn't needed !