The Spitfire round was the 30 Carbine necked down to .22(just had to look it up), so that conversion would work. The biggest drawback to the M1 Carbine is chamber pressure, it won't handle much more pressure than the original .30 round puts out, it was designed with not much extra strength to keep the weight down.
So actually, the best calibers for conversion will be comperable pistol calibers, perhaps take a Coonan magazine and tinker up the barrel and bolt to 357 Mag, or at the most do one up in 7.62x39, but that would be pushing the strength of the action, I think.
The simplest conversion would probably be to 9mm Win Mag, an unpopular pistol cartridge but one with the capability for more weight and downrange energy from a rifle barrel. I'd like to see one done up in .38 super, with the right powder, that could be a real screamer in a long barrel. I'd also like to see what the .357 Sig would do in a 16 inch barrel, that might be an eye opener.
The magazine is the problem with most conversions, but if there are mags already made, like for the desert eagle pistols, maybe those could be used. How about an M1 in .50AE? .44 mag? Lots of possibilities for someone with the resources to try it out.
Hope this helps.
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With my shield or on it...