Hi, Ron. I know about your interest in cockpit widths, I've visited your page on the subject for a reality check more than once!
I don't have exact specs, and in fact I have not yet located even a detailed 3-view of the two-seat Moustique. I don't intend to create a faithful replica, I would like to design a small homebuilt from scratch embodying the general layout, some stylistic details and hopefully capturing the spirit of the original.
Off the top of my head I would think a bench seat about 36" would do, knowing that heads and shoulders will be above the line of the top longeron in any case. That's 2" more than the admittedly very cozy Evans VP-2. That's for an open-cockpit-only design. If I wanted to leave the option open to provide a canopy or cabin top then I'd have to widen that to at least 40" or so, but given the front pylon and wires, I think open cockpit is the way to go.
I don't really see this as a two-person cross country machine, but rather an oversized ultralight with the option of taking along a friend or family member on short hops OR some camping gear to a fly-in. Most flying would be done solo.
I might even adopt the trick (used by Evans and probably others) of a central control stick, two sets of close-set pedals and duplicate seat harness attachment points. Solo, you sit in the middle, straddle the stick and use the outer pedals. With a passenger, you sit on the left and use just the left set of pedals.
Alternatively, I could take a single-seat Moustique as my model but build a tandem two-seater with a small passenger cockpit forward of the pylon and the pilot behind it. I do think the side-by-side version would easier to design, build and fly, though, because there would be far less CG travel with varying load. The original put the pilot and passenger's belly buttons almost underneath the main spar.