the hub is Maillard but not Helicomatic, right?

You can put any freewheel with the right threading. It may have French threading and that is a big issue. http://www.sheldonbrown.com/freewheels.html

maximum number of cogs is 7-8 because that is the highest count of cogs on a freewheel I've seen, otherwise is limited only by the frame spacing of rear drops (but can be cold set to another dimension)

If it's helicomatic, than use whatever you can find for helicomatic, is quite rare and extinct, so as the French threading for freewheels.