I came across the o-ring idea many years ago on this or another forum. Took my cap to O'Reilly's, they have a box of o-rings behind the counter, and got one that fit. Worked great for a couple of days until the rubber swelled from the gas and/or ethanol. So, I ordered a bag of Viton o-rings from McMaster-Carr. Been using the same one ever since (nine years) with zero problems, and I've used up all the rest on other 74-75 tanks. Don't ask me what size, just measure, it's not rocket science. This has come up a number of times, if you do a search there are other threads on it, maybe one of them gives the size.
An alternative solution would be use RTV or sealant or something to repair your old gasket, but I don't know if there is a suitable and ethanol-proof product.
Another idea I've had is to just make a cork-rubber gasket -- maybe if you made the I.D. a really tight fit it would stay on the cap. Otherwise, you'd have to glue it in place, and again, the trick would be finding a suitable ethanol-resistant glue.
In my experience, all you really need is to achieve a cap-to-neck seal. The fancy profile of the stock gasket is not really necessary.