Spent the day modifying my RVI tacho as it was positive earth and they don’t like working with electronic ignition. After a bit of research found a company that makes a board to convert the RVI tach. www.spiyda.com and it costs £42 if your wiling to install it yourself.
If you can solder a couple of wires its a relatively simple conversion, biggest challenge is getting the tacho apart due to years of rust and the bezel seal turning solid.
Movement is now converted to negative earth and calibrated so just need to find time to clean up the metal case and repaint it before I can complete the reassembly.