I think EFI on a Pi is doable... maybe a little overkill from a hardware standpoint, but doable. EFI (especially on a 919) isn't terribly complicated, but it isn't trivial either. By the time you figure out a way to interface the sensors you need with your Pi, and add the cost of extra hardware (level shifters, voltage regulators, etc), you may as well just develop your own platform and include all of that in a single BOM.
There are aftermarket ECUs on mini bikes (Grom and Z125), so I'm a little surprised I haven't seen any on larger 'budget' bikes.
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