The L & D is both 28mm, while the gearhead D is 32mm
MFA didn't supply a pinion with their gearbox? Or if you pulled a brushed motor from the box, use that pinion gear.
My late American friend Mitch Mignano had gone twice to the April Dortmund exhibition in mid 2000's & Mitch told me of the various vendors at this show that specialise in motors & gears & gearheads, ect ,ect.... I've never seen a website that an individual like me on the other side of the globe could order from

... I know of Conrads & they have a lot of specialised parts & tools, but they don't ship outside Europe. I usually just tell Frank what I want & he does my European shopping for me & sends it out.