Silly question but... change timing after swtiching ECU?

You can set the IdleTiming firing off the Negative spike,
But as soon as you open the throttle a bit, and the timing starts to advance, then all kinds of chit begins to happen, because the box cannot interpret what is going on. With your timing light wired up; you will see the index mark jumping backwards, forwards, double-firing, and missing strobes. Meanwhile the engine sounds like crap, and is impossible to drive with a standard transmission.
There are at least two (maybe more IDK) magnetic pick-ups for our Mopes; one for a clockwise rotating reluctor, and another for one that spins in the CCW direction.On the bench, you can tell the apart by the colors of the wires. IIRC the SBMs have one Orange wire, and the other does not (might be violet). The wrong one can be made to work, simply by cutting the wires and reversing their connections.
The OP did not mention changing the pick-up, I just mention this for inquiring minds.