Lots of parts involved. The Taxi and Police Force 7 digit displays used signals which would otherwise drive alphanumeric segments on the main lower display. I went the other way a year ago, had a spare Taxi and converted it for use in my Jokerz. So it can be done. You will probably need a 6118 chip, along with some resistors. The display panel schematic shows how to wire up for either sort of game (I just happened to use the Bad Cats manual and display for a reference).

Here are some notes I made for myself, no guarantees about accuracy.
Taxi vs. Bad Cats. Pull jumpers W1 W2.
18K 1/4 at R53 R58 which Bad Cats doesn't list, but the Bad Cats display has. [present, but not pulled from taxi-jokerz board]
1M 1/4 at R9 R69 which Bad Cats doesn't list. [present, but not pulled from taxi-jokerz board]
8.2K 1/2 R54 R56 R57 R64 R65 R67 which Taxi doesn't list. [Added. not already on taxi-jokerz board]
10K 1/2 R10-R13 R15-R18 which Bad Cats doesn't list. (external display) [pulled from taxi-jokerz board]
R37 R44 R46 R49 which Taxi doesn't list. Can source from unused Taxi section or buy some. [taxi-jokerz board has these already]
R63 value? given as 10K 1/2w in Bad Cats manual. [already on taxi-jokerz board]
R60 value? 18K 1/4 watt on original Bad Cats PCB. [Added. not already on taxi-jokerz board]