Admittedly, I have no experience building a motor for a land speed application, but using really light weight internals isn't how I think I'd go about it.

The parts need to be durable enough for an extended period of time under heavy load.
IMO, this is more similar to a marine application than a drag motor.
The acceleration rate is very low, so heavier parts should have almost zero affect on the speed of the vehicle.

I understand the concerns about the mains, but if a rod or wrist pin fails because they arent HD enough, it's all over.

There is going to need to be a balance found between the strength needed for the application vs how light the parts can be.