GA maintenance does not have to be done by a 145. I'm led to believe that piston GA types are difficult to integrate into the 145 system due to the cost of getting them up to the required airworthiness level, although once an aircraft has been integrated into a 145 system of maintenance the ongoing costs are more advantages, especially if the aircraft was being previously maintained under the Schedule 5 system of maintenance.
I'm no subject expert in this area, however this is what I have heard from a few GA operators who have made the change.
Last edited by Duck Pilot; 7th Apr 2016 at 22:16.
Reason: Typo