It's the original frame. They removed the housing cover and axle for the front return roller, but that's it. I added some styrene strips to recreate the original vehicles.
It's interesting seeing how they would have had to cut and arrange the road wheels and the return (teethed) roller. There aren't many good options.
I printed a version of the front axle on my FDM printer, but the quality just isn't there. I was tempted to fire up the resin printer, but I would have to add the heater (resin success rates increase exponentially when the resin is at around 78F-80F)
Instead, I scratch built it, using some spare parts from the FDM print.
The front wheels look like they're two leftover road wheels with a tire to match. The wheel I'm using is close enough.