Another option is Chris at Spiyda

https://spiyda.com/tachometer-electronics.html

I fitted one of his internal conversion kits in an old Smiths tacho's a few years ago as the calibration was iffy, but you need to be handy with a soldering iron & then have to calibrate with an audio file download

Chris is really helpful and will fit & setup for you, he's in the West Mids, UK

Jon M