Controversial take: The state should consider options for taking over ownership and operations at the plant. Maybe not as a end-to-end builder of new cars, since I believe CRRC's management design & engineering team isn't in Springfield, but the Commonwealth can 'contract' that responsibility back to CRRC for the Red Line fleet - (I'm reasonably sure that Buy America only applies to the physical article and doesn't apply to design/ engineering/ professional services), but the plant itself should operate as a quasi governmental "contractor" facility to build and overhaul transit vehicles for the T and other transit agencies, the plant could have a future - here, think Rochester shops 2.0 or the (now former) Maine Military Authority, or the current state of operations of Kinki Sharyo in both NJ and California.
The T has said that midlife overhauling it's CNG fleet in-house at Everett instead of sending them out to out of state contractors saved $73 million. Soon, the Siemens BL fleet will need mid-life overhauls itself, the early rotem bilevels, and while the HSP46s are planned, MPI has such a massive backlog that I'm pretty sure there's some opportunities there. Like MMA historically, they could be set up to compete for transit rebuild business generally from across the region, to include all the regional RTAs who might otherwise send their buses to places like Midwest. Meanwhile, Kinki Sharyo who hasn't been particularly successful in selling vehicles, has been keeping its plants alive as maintenance shops, or in the case of its LA plant, sub-contracted to Hyundai Rotem to assemble Rotem's LA Metro order.