If you’re happy to assemble a module yourself this can be done quite cheaply by buying the modules required from Amazon and connecting them together as shown here.
The module I have been using for initial testing (and therefore the one I’d recommend is the HM-10 module… I’ve also used the HC-08 module too which also works. The two are pretty much interchangeable.
I’ve bought a few for different projects and the DSD Tech ones have been 100% working so far, out of the box. A couple of the other (cheaper) ones have failed to advertise when powered up and I’ve not spent any time trying to figure out why yet. The DSD ones come ready set up at 9600 Baud (Suitable for MS1 and MS1/Extra).
The module provides an interface between a serial Bluetooth stack (that the iPhone can connect to) and “UART” which is a low voltage serial connection. To connect this to your MegaSquirt ECU’s serial port you need a suitable adapter such as the one below. This plugs into the MS ECU and provides a bridge to the Bluetooth module.