On Saturday, I fitted the tank internals from my 1150 in to a GSA tank.
You'll need a load of small Jubilee clips. I used the type with the screw across the two faces rather than the worm drive type. size wise, mine had "11" stamped on them (11mm?).
Unbolt the round plate from the tank & disconnect the two hoses that come from the filler neck. Remove round plate, pump & fuel filter as one assembly. Note which hose goes where, then remove filler cap & filler neck.
Fit filler neck in to the new tank, the filler hole is oval with the oval at right angles to the tank & the plastic assembly attached to the filler neck pointing towards the rear of the tank. Stick hand up inside tank via round hole & pull the two hoses in to place, insert pump & filter assembly & attach the two hoses to the relevent spigots on the round plate using new retaining clips. Very lightly grease the large O'ring before tightening up the 5mm nylocs - use a tiny dab of copperslip or similar on the threads as they're prone to rusting. Don't overtighten them, I used a nut spinner only.
There's an O'ring beneath the filler cap/neck assembly, buy a new one in advance as they're prone to stretching & impossible to fit back in to place. They're only £1.06 new. Re-fit tank as normal. You can stretch the O'ring over the top of the filler neck, you don't have to disconnect hoses etc.
My GSA tank was bone dry, I put exactly 5 litres in & rode 1 mile slowly to the nearest fuel station where I managed to put another 27 litres in? Mind you, I'd already drilled a 6mm hole in the filler neck that allows trapped air to vent & another 1.5 litres of fuel to be squeezed in.