I went and pulled the head off my 91.  The PO said it was a pretty new top end on the motor.  Well in my novice opinion, it looks pretty good.  The cylinder walls appear to be nice and smooth, there are some  vertical lines, but I can't feel them at all when I drag a finger nail across. The top of the piston is marked reading from intake to exhaust side:

1  50mm
652 P6
CD

Not sure what brand of piston it is.  My concern is, there appears to be movement side to side when I put pressure on the top of the piston. I am not sure how much movement is okay, but on my other bikes if I recollect correctly there was not movement between the piston and cylinder walls.

The inside of the head looks good and there was little carbon build-up on the top of the piston.

Not sure how to proceed. I am going to pull the jug off in a bit, as I have the billet washers and RM bearing to install. Not sure if the pictures are going to show that much. But it's worth a shot.

I notice it at both the top and bottom.  I cycled the kickstart to move the piston to both TDC and BDC and the movement is the same.  So with a little cleaning using a paper towel, the cross hatching is very apparent and clear.  There are no grooves or vertical scratches that I feel.

So I have the jug pulled. Man this thing looks good all the way down! It is way cleaner than expected.  I was planning on sending it out to have the squish set anyway, so might as well have it checked over to make sure everything is within spec. I remember on my 500, it had a fresh sleeve in the jug and when I sent it to Mr. Hall to be o-ringed he found the bore was not true and had to clean it up. I guess with something that has already been run and bored previously, you have greater chances of not being completely round?

Based on the numbers on top of the piston, can you tell me how many times the cylinder has been bored or what size it is? I am assuming this is a wiseco piston as there is a w inside of a circle on the underside? Just guessing though.

There is no play in the connecting rod. I have side to side movement when sliding in the bearing, but nothing up and down.

I went to measure the inside of the cylinder and the dang battery on my cailiper has died. Need to replace soon, as I use them when reloading shells. Coyote season is upon us so can't be without bullets!!!

Yep, 67mm is stock.