None that I know of - the seat rails on a DT1/RT1 frame split out from the main backbone quite a long way forward, meaning that any replacement tank has to have a very wide tunnel at the back.
The (rare) DT-MX tanks have the nicer screw-in cap, if you can find one.
The other option is to cut and shut the filler from any of the steel tanks with a screw in cap.
All of the steel tanks are stamped with a hole for the filler neck, and then they weld in which-ever type of filler neck (screw in vs trap door). This welding it done from the inside, before the inner tunnel is welded in.
So if you've got a donor tank with a screw-in filler neck (red-tank MX100 or monoshock DT are probably the most common/cheapest donor tanks), then its easy to cut the top off the tank and grind out the filler neck.
Getting the old filler neck out of the tank you want to keep is a fair bit harder though... If you think of a better way than hours with the die-grinder, then let me know! Actually... I've got a 90* drive for the Dremel - that would probably be a lot easier/quicker and you'd only have to grind away the weld, not the whole filler neck.
Anyhow, then braze the new filler neck into the original tank, because you can do that from the outside without having it look poop-house.
The screw-in filler caps are available new, for ~$40 BTW. They seem to attract silly money on Ebay at times, presumably because people don't realise that they can buy 'em new from Yamaha for less.
