150mph speedo?

Electronic and mechanical are 2 completely different animals.
A instrument/speedometer service can convert your standard mechanical instrument and calibrate it to a 150, or 200, or maybe 1000 mph screen. They would take it apart, alter the fly weight of the drum and /or change the strength of the magnet inside the drum. It wouldn't be cheap. If the instrument has significant wear in its spindle, etc.. anything short of complete restore would be a waste of money.
Poor boy method... create a custom screen and change speedo drive gear at trans to get it closer to accurate.