Complicated, I understand that the start -up (starter engine) is 24 and you want to make it work with 12, it would directly have no strength. Options:
a voltage converter that elevates 12 to 24 and put it in an independent circuit, only the converter output to the starter engine, but I do not know if something of that is sold and it must also be of great power, there are 12 to 220 alternate output; But you would have to go from 220 to 24 and make it continuous, I also think it would not have enough power because they are 250 or 300W.
Bobinate the engine to be 12 (it would be too expensive)
that the characteristics of the winding allow you to disapprove and weld the coils with another type of connection (parallel) so that it works at 12, consult an electrician.
I can't think of more ideas. Luck.