I think that the only “much later tanks” that will fit a vintage TT Replica will be those made for Reps ONLY in the period 1931 to 1934 when all the Flyer models of the era had the single downtube frame. The G.P. Scott models were also the equivalent to the Rep., and so I include those. GP tanks can usually be identified by their CMC ( Coventry Movement Company) tommy bar type filler caps and matching necks. HOWEVER, I do have a mystery tank that has these fillers, like a Brough Superior, yet it also has a cylinder wall oiling pump on the underside, like a Rep tank, AND it fits a vintage frame……
I intend to fit it on 1928 TT 1, as and when I get around to starting that rebuild, as it is VERY similar to a 28 TT ‘Works’ tank, except for the CMC fillers. Looking through period photos of vintage Scotts, the only bikes I can find with those fillers are the long tanked Sprint Specials, so I am fairly convinced that my tank came from the Competition Department. Perhaps they had a few CMC caps and necks left in stock when they produced the very small number of GP bikes, and decided to use them up.
The 1939 Clubman Special tanks were different again, and wider than a Flyer tank, but Flyer tanks 1935 to coil ignition/separate oil tank models in about 1948, were all pretty similar and interchangeable. Due to their propensity to oil leaks many Rep tanks have had their cylinder wall oiling pumps removed, and so get confused with standard Flyer tanks.