yep, we’ve been thinking about petrol vectored tailsitters too. We already have code in ardupilot for automatic engine start:
See this example:
right now it can only auto-start one engine, but we could extend it to two. It even restarts the motor in flight if it needs to. We had that happen on an auto mission. The motor stopped (bad fuel supply system), and ArduPilot automatically restarted it and it continued its mission with no operator intervention.