Couple of questions:

Does it bust at 20 for both 1 player and 2?

Yes it does .. should have mentioned that .. also note that increment outputs of each card score reel are connected together (player 1 10's to player 2 10's and player1 1's to player 2 1's)

The scanned diagram cuts off... what happens on other side of PLAYER RELAY ... BROWN wire... where does this go?

I've scanned more (see attachment)

Do 20 point relay AND bust relay activate at the same time and on 3rd increment.
Yes, both activate at the same time when card score reaches 20 for either player (either 10 to 20, or 10 thru 11 etc to 20)

Im really struggling to find a path to ground on 20 pts unless the BROWN wire takes us somewhere.

I've also started looking at where the brown wire goes

Also check that on "A "Player relay.. normally closed switch is GREEN with RED on one side and YELLOW  with BLUE  on the other... this one goes back on itself to a single leaf switch onm same relay

Will check that shortly and advise