To work this out properly I'd need proper specs on the motor concerned, a scale drawing of the whole system, and to do the maths properly, which honestly I can't be bothered to do

Rule of thumbing it, mod2 would easily work if the gears aren't pot-metal, I have no idea on the specs of these so can't say but they probably are fine and if they don't you can get some better gears from somewhere like ondrives.com (expensive though!).

Best bet though is to go for the chain drive! The reason why is that to get gearing to work properly you need to work fairly accurately, significantly better than +/-1mm, whereas you have a bit more leeway with chains - as (I'm assuming given that you're using wood and such like) you don't have access to accurate machine tools it'll be a bit of a pain to do the mechanism right. A chain drive reduction is a lot less finicky about clearances and everything being dead straight. In addition, unless your structure is extremely rigid (and therefore heavy and/or hard to make; a milled aluminium gearbox setup would be your best bet here), if it gets tweaked during a fight, say if you get hit hard, your gears will probably bind up.

Alternatively, there's the expensive way of doing things, which is designing the mechanism, accurately drawing it up (CAD is an option here), and getting it made at a machine shop.