Hi I'm looking for a good usable fuel tank for the above bike
Any clues to where I might find one?

as rare as that then?

Can't help sorry, except i was initially reading it as YG1 [dyslexic me probably] which is about 1968 80cc model [YG1 name]. I guess 1G1 the model code which i should have realised & images looks DT175. I'd guess any 125/175 tanks off those models would interchange through the monoshock frame years.
www.partzilla.com/catalog/yamaha/motorcy...978/dt175e/fuel-tank

1g1 is UK Europe model designation, twin shock not mono!

I have a 1G1 DT, too, the tanks are very rare, I am trying to resurrect a really rusty one at the moment,
also the 100, 125 and 175 tanks are not all interchangeable, there are differences in length from the rear mount to the front mount stubs, I have read that the 175 and 100 share the same tank size, and also read the same about the 125, but I know of at least 1 instance of a 125 tank being too long for the 175, on my profile I have uploaded an image of my tank repair,

The part number for the 1G1 indicates it was first used on the DT100C, which was never imported into the UK back in the day. The 76-78 UK only 1N1 DT100 and the 76/77 European 1F9 DT125 used the same tank. Tanks for European/UK DTs are certainly getting hard to find, along with many other parts; it took me 4 years to find a seat for my 73 DT175 CT1! Have you tried DK in Stoke on Trent for a DT100C tank, as they seem to import all manner of DTs both as projects and for breaking?

Found one

Congrats!