I'm not a carb expert. As mentioned before, the later XR2 had a 28/32 carb. If it's a 21/23 carb it's far too small.

I just use Reinz or Goetze head gaskets. Payen seems to be a bigger brand, for sure better than the first no-name one on ebay. Buying as a whole set with valve stem seals etc. is cheaper.
And yes, you should always use the later "hat type" valve stem seals.

I wouldn't mess around with changing rings on two pistons... either do all (and then check for ring clearance, piston clearance and use new bearings AND bolts) or none.

On such an engine you can usually use any exhaust manifold gasked you want to use. Thin sheet metal type, sandwich type, round exhaust ports, square ports... won't matter.

Preventive things? A full rebuild maybe? *lol*

No, it has a carb, therefore the are no fuel injectors.

Depends on the age of the head. When it's off, check for the cast markings on the back side. ALL CVH heads from January 1985 on can be used with unleaded fuel. Heads cast until December 84 will have "LPG" stamped near the exhaust port of the first cylinder IF they have hardened valve seats.