Hey Patrick. I haven't tried 50mm fenders with MiniMotos, but I've tested the limits of their clearance a bit. Here are some examples:
- I can confirm they technically, but not safely, clear a 650bx2.1 Thunderburt on a custom fork (brake posts were spaced for Honjo 62 fenders - Minimotos do not clear these fenders)
- I can confirm they clear 48mm tires without fenders, same fork
- I can confirm they clear a SKS B5 fenders on a Quickbeam.
- Closer to your sizing/situation, please see photos below of two different bikes with Minimotos:
This is a Naches Pass w/o fenders on a '92 XO-1:
And here are photos of a Quickbeam with 700x38 GravelKings, with 45mm Smooth Fenders from Velo Orange (that might look close, but it fits nicely, no clearance issues):