Well, the mag would really depend on the gun, so better to address that first.
Yes, you can probably fit a Crosman clearsoft gun's internal in a MG42 shell. However, that MG42 shell has better already be in Canada. Otherwise, you'll find that importing replica firearms is a bitch.
In the event of access to a MG42 shell in Canada, you'll probably want to stick something better than a Crosman internal inside it. Those clear Crosman guns essentially represent the bottom of the airsofting rung, and might not even survive such a transplant.
"The Bird of Hermes is My Name, Eating My Wings to Make Me Tame."