260 to 290 is richer in fuel. That should have not helped the fouling.
As for the loose spark plug. That is an iffy question. I have had a plug
back out and it did not toast the piston. I have heard of other people say
that it has happened to them though. My thoughts..... Its broke now so fix it.
Install forged piston. They are better then cast. I have toasted 2 cast pistons
before.
The porting will be a big difference. Make sure you use a good shop for this.
There are different types of porting also. I mean different steps. Play, Drag ect.
For your first Jetting after porting I would let a shop get you close. That is a lot of
money to blow on a guess. After porting mine for Play port, I installed 330's.
My carbs were stock and not borded out.
Good luck.
Supergrzz