I have a Zeiss "Donata" 9x12 plate camera (c. 1930), which is a descendent of your Maximar.
I found a RADA (Plaubel) 6x9 roll-film adapter for 120 film on e-bay this fall.
I have shot a couple dozen rolls of 120 with this camera, and love it !
Other makers were Suydam and Rollex, to name two.
Be careful about Rollex: a friend just got a 9x12 Rollex film back that turned out to be for 116 roll-film, long-obsolete here in the US.
The Kodak Recomar 33 is the most common "US" 9x12 (although it was actually made by Nagel Camera Werk, which was purchased by Kodak in the '30s) - Recomar film holders will also fit the Zeiss cameras.
My Donata came with three plate-holders; I need to find either film-septums / sheaths to use sheet film in the plate-holder, or actual sheet-film holders.
Good luck with your Maximar !
Luddite Frank
PS: while these cameras did have a ground-glass focusing-back, it's not a backwards-displacing spring back like a view camera or a press camera, where the film-holder slides-in in front of the ground-glass. The ground-glass back has to popped-off the camera after focusing and replaced by either the film/plate-holder or the roll-film adapter.
(Kind of a bother)
I've been using my 9x12 as a "hand camera", using the fold-out wire-frame finder, which I've stretched rubber-bands across to create frame-lines for the smaller 6x9 format. My camera has a 13,5 cm lens, which is a little bit "telephoto" for 6x9, but I've had good results as far as framing goes. I estimate focus distance and try to stop-down as much as possible, to gain maximum help from depth-of-field.
I use a Gossen Luna-Six meter (CdS), and Fujicolor 160 print film.
I mostly shoot landscapes, railroad scenes with mine.
You may want to seek-out a lens-hood, as these pre-war cameras have uncoated lenses, which can be prone to flare.