Is there a comprehensive guide here, or anywhere online, to repair existing cracks and to reinforce? In others other words what can be done?
I did try the search function and got scattered posts on this subject.
I installed the UAV Bits reinforcement plates a year ago. So far, no motor mount cracks.
They were cheap, easy to install, no disassembly required and came with the required longer screws. Easy peasy.
The plates distribute the load better at the motor mounts and should slow any cracking in that area.
The big deal is their design
eliminates (IMO) the possibility of cracks between the motor mount screws (if they occur) that sometimes allow a motor to separate from the drone in flight. That just can't happen with reinforcement plates installed.
These plates obviously will not prevent cracks in other places, but most drones do not get them. It appears, however that many do. Mine is 2 years old and has no cracks.
To your question about repair costs - it's several hours work plus a $79 shell. If I saw serious cracks forming, I would just replace the shell. It is a hobby, right? It might be fun. A commercial user would probably just send it in for repair. But with labor costs, it might make more sense to look for a new or refurb replacement aircraft.
Hope this helps.