The engine's irrelevant, the speedo is getting it's reading from the box I assume, the std type 9 place ?
The oe speedo will have been made with a certain turns per mile Known of the output from gearbox and the rear axle/tyre size.
Change anyone of these and the speedo will need recalibrating, around £50, speedy cables in wales do this.
Just need to give them the gearing details of top gear of box, diff, overall tyre size/turns per mile, and they'll do the rest.
tabetha