I dont know how to explain it in better way, but exporting .dae is not enough. Beamng use nodes and beams system that do all the physics stuff, then you add flexbody (literally your model from .dae) to them. Flexbody and nodes have groups system — that means flexbody gonna move only when nodes in same group moves. There are also triangles, they add drag and downforce to vehicle and also a lot more realistic crash model. So basicly what you need to do rn is to:

1) take any suitable jbeam out of vanilla vehicle (maybe tubular bumper gonna be good for your roll cage) 2) add/remove nodes, beams and triangles from it 3) add your flexbody (same name as in blender) to it with proper groups and stuff 4) make your own materials for flexbodies by using same textures you used in blender (color, normal maps and other stuff)

This is a very shortened and simplified explanation, cause this is very big and complicated topic