Hi!
Get a small 9V–0–9V transformer of about 5–10VA that will fit in the unit or on the PCB and use one of these kits for the high–voltage supply:–
https://www.ebay.co.uk/itm/High-Volt....m46890.l49286
The Operation Manual for these is also a Service Manual with schematic diagrams and parts lists, so you can use the information in it to help you get the correct h.t. line voltage for the display!
I note the RTW 1206D has a metal tubular stand – has this a hollow space you can use for a small 9V transformer and the PSU kit I suggest?
Chris Williams