I also had some ribs and running board supports break off on my Cookie Box Yarmouth Kit. Too make the repair I did the following
I used .010x.020 stryene for the ribs. I started by cutting out the the bad section and lightly sanding where the rib was for a flat surface. I made sure I left the rounded outside edge of each rib closest to the roof edge I Cut a piece of the .010x.020 styrene to fit and holding with tweezers brushed on the bottom of the new rib about 3 light coats of Tamiya Extra Thin Styrene cement to soften the plastic. This make it sticky and I next brushed on a coat of the Tamiya onto the resin roof where the new rib goes and carefully placed the still still softened rib in place. I did this for all the ribs and after they set up I then applied a bead of Extra Thin CA along the seam. After dry I lightly sanded with a 320 grit sanding stick. I know this sounds like it should not work, but it does.
I also used Moloco running board supports. I stated by just changing a few that were broke, but ended up replacing all of them. Not totally happy with my results as I should have made a drilling jig, but once the running board is in place I think it will be OK.
Attached photo show almost ready repair. Still need a bit more sanding on ribs. Alos needed to replace part of the upper door track as well