Did the U.S or other allied powers provide support to China in their war against the Japanese in WW2? If so what kind and to what extent?

Yes, the US supplied the Chinese with hundreds of thousands of rifles, including 107,470 Springfield rifles and 152,241 M1917 rifles.

Furthermore, the Canadian Inglis plant would build 7.92mm Bren guns, 7.92mm being the standard ammunition of the Chinese Nationalist forces.

Ironically, these weapons would be used against their manufacturers in the Korean War and the Vietnam War.

I strongly recommend you check out the first source, it has the numbers on a lot of items supplied to China.

Sources:

The Bren Gun, Neil Grant