Hi!
Without going to "valvephoolery" prices, the OP can get a reasonably priced EC91/CV417/6AQ4 from this link:–
https://www.ebay.co.uk/itm/Mullard-K....m46890.l49286
The EC91/CV417 should work if the leads to pins 5 and 7 are swapped over, and any wiring to pins 1 or 2 transferred to pin 7, and anything on pin 5 transferred to pin 6!
ECC91 (original) base:–
1:– a', 2:– a", 3:– h, 4:– h, 5:– g' 6:– g", 7:- k
EC91/CV417 (suggested):–
1:– k, 2:– g, 3:– h, 4:– h, 5:– k, 6:– g, 7:– a
Chris Williams