I assume you're asking two questions:
1: How to adapt a MegaSquirt to your harness, and
2: How to adapt a 1.8 engine to your harness.
1: You have several options here. There's the MSPnP
which is a turnkey that plugs into your stock harness. There's the DIYPnP
, which you have to build, but is cheaper, more powerful, and also a direct plug-in. You could build an adapter cable like This One
, or just do what I did and cut the factory connector off and install a DB37 directly.
2: Electrically, you have a few choices. You can move the electronics from your 1.6 engine onto the 1.8, or you can cut a few ends off and install the appropriate 1.8 pigtails. Depending on the year of the engine you choose, variables will include the ignition coils, temp sensors, TPS, crank/can pickups, and probably a few other little things I forgot.