No....you can't switch mpem...
I studied your electrical drawings last night in detail, just didn't make a post. In every application of any engine, you can apply a tach......even on a lawnmower engine. The tachs impulse line runs off the pick up/ trigger coil of the magneto, that tells the engine when to fire in sequence. Problem is, whith your rectifier wires being all the same color, I thought is might be your grey/white wire coming from your pickup coil. But here, your taking a chance. BUT, if you look at the wires coming from your magneto pick up/trigger coil, there is a BL/YL wire and a YL/WH wire.
These wires send in impulse to the mpem so that it can deliver the data to the coil. A rpm gage works on this circuit, in any applicaton. Whether its a car, motorcycle, plane.........doesn't matter. RPM comes from the electrical signal that pops an impulse each time the crank makes one revolution.
The only problem, is I can't determine which wire you'd need to tap into from your magneto.
The other thing I'd point out. Your mpem does have the rpm signal going to it from one of these wires, I checked. The mpem is what uses that signal to keep you from over revving. That's how your rev limiter works.
I don't see a need for you to have a need to know what your rpm is, while the engine is running. You can not over rev your engine. It is protected by the rev limiter of your mpem.
But if it's a must, I'd try a connector from the YL/WH wire. If you connect the purple and black up the way it's suppose to, there shouldn't be any consequences if its wrong, but to blow the fuse.