Again, yes and no - the driver is the same, but only the original K240 was the true sextett: it has 6 passive diaphragms around the main driver which essentially improve bass performance. Also the K241 was the last sextett style headphone with 6 vents instead of diaphragms. Also they are circumaural (around the ears) unlike the K141 which is supra-aural (on the ears).
I don't know where to get the elastics, but on the K141 you can also just buy small/thin black girls hair bands. They work absolutely fine and are much cheaper. I also just DIY the inside foam with cotton wool or spare foam I've got lying around.
As long as you don't use something extremely different from stock, the sound doesn't change much.
If you contact AKG they will point you in the direction of a stockist of older parts in your area, and usually they're not cheap.