1st Gen remote starter

Rather than identifying a remote start that is designed for the Prius, the task is to figure out the conversion logic required to use an ordinary unit. When I went shopping for a unit for my 2006, nobody would accept the job. The classic should be easier than a 2006, but I still doubt that you'll be able to find someone to handle it.

Conventional remote start systems want to turn on the ignition, run the starter, and watch for a RPM response from the engine. If the engine doesn't start, then the system disables the ignition and retries up to 3 times.

You've got to satisfy the remote start's need for an "engine running" signal, even when the Prius just goes to Ready mode. I found that when the 2006 enters Ready mode, the 12-volt system goes up to 13+ volts, so a voltage sense may serve the function. Before I discovered the voltage change, I made a custom circuit that generates a tachometer type signal. You may end up needing something like that.

On a 2004+, the push start/stop is a complication that will also need extra logic.

You'll need an extra key and a transponder bypass unit to get past the transponder security. I wasted $40 on a no-key transponder bypass for my 2006; the KEYOVERRIDE-SL unit is documented to work, but failed for me. I'd guess that a simple universal bypass unit would work. You get an extra transponder key, and mount it inside the bypass unit. Again, I tried this with my 2006, and it failed.

It's getting cold, so I'll probably be working on this again soon. This is a solvable problem, but apparently a do it yourself project since nobody wants the task. Good luck, and let us know about any progress.