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Any advice how to fit the fuel tank brass collar/ any fitting kits etc?

I firstly fully screw the cap to the brass collar so that I can mark the collar and tank filler neck so that I can position the cap in the orientation that I want. Then remove the collar from the cap, align the marks I've made and tap the collar onto the filler neck using a hyde mallett.
I also install a suitable O ring between the filler neck and the 'step' into the brass collar, this helps prevent leaks, and means the step in the collar isn't tight up to the filler neck. It gives just enough room to get a screw driver in there in the future to pry the collar off again if you need to remove the tank. I also have a thick paper gasket between the cap and the brass collar.

Certainly no tapping it on.. I’ve heated it up with the blow torch and no good. I’ve boiled it in water and still no good.

If the diameter is too small to start with, you'll have to machine it until it is a .001" or so undersize. Then heat it up and slide it on.

Probably best to find out why there is an issue .. so "measure" the Collar internal & Tank Neck external dimensions.. dont have measuring calipers. ..then apply some type of marker pen or "engineers blue" to the contact surfaces to then see where the high spots are within the Collar to then be able to sort a fix .. such as using a Scraper to remove high spots or maybe an internal skim using a Lathe or even the Dremel & if you take too much off then there is Glue

Don’t use a hammer like I did and mess up the threads

If only I could get them off again!