3d models for the game can only be exported using 3d Studio MAX 5 with an exporter plugin. It's the only known way to create the needed ACT files. I'm not sure one could use Blender to create it and then import the model in 3d SM to export it to that format. In any case, 3d SM 5 is needed.
As for the crossbow it would have to be a static model, adding new anims is almost impossible.
The Heli would suffer from the same issue as in BE5, it would still only move fast while it travels on roads. The whole system is based on the global map TGA file which has a layer that determines speed of movement. The code for it is not available, so it isn't fixable. In BE5 we couldn't remove the random encounters either, but that can be done in 7.62, but is it worth it? I think the 3d model for a Heli is already available in 7.62, several maps has them already so it would be a matter of creating a new OBJ file for it.
But since the only thing a Heli would bring is the removed random encounters, and maybe a "coolness" factor
I would vote against it.