Then you'll either need to fit a suitable sensor near a wheel, ( or driveshaft, or propshaft etc etc ) watching a suitable toothed trigger. Whether that's the disc bolts, an actually toothed wheel or other, up to you.

Or slightly easier retaining the speedo cable, something like this should cover you. You'd need to enquire what sort of fitment you need for your setup. But a few places make similar stuff for the likes of the Terratrip, rally stuff.

https://www.ebay.co.uk/itm/144297421877