As long as the upper comes into Canada as an upper only then it would be an uncontrolled part in and of itself.
1. If put on a restricted firearm, ie. semi-auto AR variant then that firearm remains restricted.
2. If put on an unrestricted firearm and still maintains the overall length greater than 26"(or whatever the metric equivelent is) on the host gun, then it remains non-restricted. If shorter, it is prohibited.
3. There is no magazine restriction on 22 long rifle rimfire magazine devices. Therefore, no restriction on the belt length.