420 chain - 420 sprocket. 420 and 41 have the same pitch, roller diameter and width (the internal pin is a different diameter) and use the same sprockets. 35 is a much smaller chain. 40 has the same pitch as 420/41 but slightly larger diameter (+.006") and wider (+.0625") rollers. A 40 sprocket would work if it were thinned (machined to be thinner), but it would be like starting out with a slightly worn sprocket since it's machined for the slightly larger dia rollers. There are sprockets marketed as 40/41/420, they would be machined for the larger dia 40 rollers but for the narrower width of the 420/41; like most one size fits all products, they aren't ideal for any size (they are just a thinned 40 sprocket as mentioned previously). The .006" difference isn't huge, obviously, and it would work, it just wouldn't be ideal. A 420/41 sprocket would be ideal.
The bike you're getting has a huge rear sprocket, much larger than the 53T sprocket on the CVT equipped Pro Series 212 (look at photos and compare). If you use that sprocket with a CVT, which adds a variable primary drive that starts out at 2.7:1, you will have some serious gearing. You could be a hole shot hero, if you can hang on to it. You could compensate by using a larger than usual sprocket on the CVT, but I don't know what's available.
After all of that, I just looked at parts diagrams/lists on the MM site. They list the Pro Series 212 chain/sprockets as being #40; I'd bet your bike will have a #40 chain, also (it would make sense to use the same chain on both bikes). When you get your bike, check for a number stamped into the chain side plates. If by some slim chance none of the plates are stamped, measure the roller width.