I've no longer got the details, but the main mod was to use the narrow pad (used when that particular tube went from 9 to 0) on the Nixie score mini-selector board to pull-in a relay, which then incremented the next decade. Originally, that pad drove the next decade directly, taking much more current and eventually burning-out.
Some other mods centred around the micro-switches on the pops, which had a habit of sticking and locking-on the coil; can't remember what I did, though.
On Flower's Child, the star rollovers in the right-hand channel powered powerful magnets to propel the ball up the field. I used those rollovers to drive power transistors instead, which made the thing more reliable.
These Rally machines were extremely unreliable on site and operators hated them. Many machines were unceremoniously thrown away, which was my gain as I collected them for zero cost. Unfortunately, I had limited space and was able to take only about a dozen, leaving many behind! More unfortunately, I got rid of the lot in the 1970's!!!