Good news: there should be nothing that needs to be done to the loco chassis.
The clearance above the top of the Drive Unit, in the region
shown shaded in the diagrams below, is a little less than the
1.5mm maximum deflection (0.75mm primary + 0.75mm secondary), which is the arbitrary limiting design criterion
that we normally work to. Our design maximum service deflections are 0.5mm + 0.5mm, which should
be accommodated comfortably. Even these are
probably far in excess of what will normally be encountered, matchstick-climbing stunts notwithstanding.