True enough, only USA and UK and Free France units were all motorized (and mostly thank to the USA and UK production).
What of above of the Soviet army is very true. Soviet army lacked trucks - most of the lend lease program they received was not about planes and tanks, but food (yes food! the Red Army was fed by USA imported goods), rails and trains, trucks, etcetera. They also received tanks and planes - but a think is to drive something to ferry stuff, and another is to operate a foreign tank (maybe with foreign language written instructions)
Considering there is OIL as factor for motorized forces, I think it can be simple as hell to add motorized units and regular infantry units. But it depends how complex the game is to be. There are old games such as Third Reich that simply has infantry and armoured units. (And motorized formations are either bundled in the armoured units, as in most cases for germans, or are infantry with extra movement, as in most cases for allied forces).
But in general it is premature to shape judgements before to get a pulse of the game.