Your kart would be just fine with a 1-1/4", or a 30/40/50mm axle.
I'm not sure what diameter axle your kart would have originally had, but I'd suggest you email Margay and ask:
http://www.margay.com/company/contact.html They're very friendly/helpful and will give you all kinds of other good info like what length axle/wheel-hubs it had, the recommended/default ride-height adjustments, toe-settings, exsettera to make your Margay model handle, turn and work as well as possible.
I'd just ask what type axle they would recommend, and then also ask if they could include any other setup-info/specifications they might still have for your model. (find the sr# near your r/f spindle and include this in your email)
You might also ask if they have a good-used complete axle assembly for sale. (and any other parts you need)
If you used all genuine Margay parts, your kart
would end up with more resale value...
Although pretty much all parts besides possibly the brake caliper and front spindles will usually interchange between most all brands of Sprint karts like yours.