On the 60C/70C, the tacho runs off a 2-pin male/female plug/socket, on a fly lead which plugs into the main loom near the control box. There's a separate loom from the motor for the trim gauge.
Does the loom on your new control box have the round 8-pin connector on the motor end? If not, then you'll probably have to change the entire loom. Not a difficult job, provided you have the parts, of course.
As 9D280 posted, what was wrong with the original?