I would prefer to think of this as a “mass conscription”, since at this stage the American contribution was largely in manpower, certainly not in advanced weaponry, which it had to borrow from the Allies.
This goes back to something I mentioned before about the UK & US not having compulsory military service; hence very small peacetime armies. This did mean, however, that upon declaring war they could enact conscription which would immediately raise large, but untrained armies.
Britain did not do this immediately, relying on volunteers to swell the ranks of the small professional army. But as casualties mounted up, the Military Service Act brought in a flood of new manpower:
America was much quicker to bring in conscription:
Therefore, I suggest that when America goes to war, rather than getting an IPC boost as such, it gets to declare a draft and can place X “free” infantry. These can perhaps be trained up into other units, though I prefer them having to be shipped to Europe to be equipped with B & F weaponry to bring them up to standard.
Britain could be given the same option once per game, but there would have to be a balance (loss of morale?), or the declaration restricted to turn 3 or later.