Motorbike turn-signal blinker starts OFF, I want it start ON
The main issue is that the circuit starts off "dead" and MUST charge a "local" supply before it can turn the FET on. IF you can bring in "true ground" then it can be done "with ease".

Try reducing C1 to as low a value as will still allow proper operation. Startup time is in part related to C1 charge time. Also implementing suggestion 2. below will minimise the oscillator time to input low, which is needed to turn the FET on. 

Suggestions: 

I am not going to try to 'work these out' in detail without knowing if (literally) chassis ground can be provided or if an NC relay can be fitted in, but the following 'lines of attack' may be profitable:


If you can add a relay with a NC (normally closed) contact it could be used to assist startup and then be disabled by a driver. The IC numbering suggests that you have an hex Schmitt inverter package available. If the other gates are available they could provide the required logic. 
Returning C2 to Vss rather than V+ MAY help somewhat. 


3 Placing a resistor between the two inverters and a cap from inverter 2 input to V+ MAY help somewhat. This gets the first inverter driven on initially.