Hi RGM, The sleeve you see in intake port is normal. It's a hollow tube they put in the mold when they cast head. The head bolt goes through it.
Loose guide in head explains a lot. As Koan stated oversized guides are readily available. The machine shop can/will custom turn/fit the oversized guide. You need to find machine shop skilled in Triumph head repair to have it reamed & appropriate oversize guide installed. Be very careful of the shop. If they ream guide bore crooked they have to grind the dickens out of the seat to get it Concentric
. This will ruin seat. This is routine work for a skilled shop with the proper equipment such as a Serdi head machine or the like. These heads are gold & hard to come by. Don't let anybody mess it up. Advice on best shop will have to come from UK members that have good personal experience with such shop. It will not be cheap.
Regarding pistons, rings, if needed you can remove cylinders & look at pistons & rings. If all ok, simply blow off rings with compressed air or wipe best you can. DO NOT REMOVE RINGS!! If ok, you can simply put cylinder back on. Rings will find their old places in a few miles. I've done this hundreds of times to good results, like replacing base gasket or resealing tappet blocks. A pair of real ring compressors & piston boards to hold pistons makes this a very simple task. Lots of ways to compress the rings, but the real tool is a gem.
If you remove rings all bets are off. Often then the rings will slightly distort & it will burn oil & smoke.
If bores look good, you may well want to just leave cylinder alone. Get head repaired & installed. Then reevaluate the smoking. All you'd be out is gaskets & time.
It is very sad to see what some owners do to these bikes. It is said these bikes are simple to work on. Compared to a modern twin cam 32 valve V-8 with cam adjusters they might be. But they demand precision work & every thing done just so. Then they really do work good & can be quite trouble free. Very reliable.