With a modern Reg/Rec unit like the Trispark, voltage is maintained and handled better than the poor old Zener can handle if not using the higher wattage original bulbs. Maybe mine was faulty and just coincidentally died months after going to led headlamp/tail/brake. The positive earth zener's are pretty much unobtanium as replacements these days, so no real choice to swap it like for like.
I think I'm wrong about an LED warning lamp as an alternative to tweaker....sorry.
The turn warning bulb is a single bulb on our bikes, which passes current thru when either left or right turn switch is closed. With LED bulbs in the corner lamps, the warning lamp draws more current than the turn led's and so stays lit, not flashing like it should and both left/right lights flash together. The tweaker is a couple of diodes placed on the connections from the left/right switch circuit....so current can only flow through the warning lamp one direction at a time.
View attachment 117059
Another option I came across but have not tried, is a special 3-pin LED flasher unit with a 4th pigtail that can control the warning directly....available for both +ve and -ve earth use:
For Classic & Modern Cars & Motorcycles. 12 Volt 2 / 3 pin electronic indicator / hazards. Keeps flash rate constant for LED & Filament Bulbs. 64461334211, 644.613.342.11 Negative earth 502096. Positive earth SFB100
www.classiccarleds.co.uk