After some time of consideration I bought a 3D-printer. Unfortunately my CAD-abilities are limited so currently I design rather 'simple' parts (skirts and tank for the RP210, ventilation hoods for the Zephyr baggage) which on the other hand would be quite difficult to manufacture w/o the printer.
I bought a 3D-printed 1/29 kit of a PRR BP20 A which I will modify in some areas (trucks will be donated by an USA-Trains PB). I'll try to build a B-unit, too.
A somewhat generic vintage cab-diesel is available on thingiverse for free - it resembles an Erie-built passenger unit and is designed in an unknown scale. With some scaling, re-arranging etc. I was able to 'extract' the cab-area and have it printed. It will need some modifications to get close to the FM-prototype but it is a good start - I'll use an USA-Trains PA for trucks and chassis. I already prepared the CAD-data for the rather spherical roof.
I hope - with some more experience in CAD-design (I use FreeCAD) - to be able to design some more unique (early) diesels. Also the passenger cars to the RP210 would be feasible - but this will require a quite steep learning curve.