Can a US registered motorglider be flown in Canadian airspace when piloted by a pilot with the appropriate FAA glider license?
Unless your license says on its face that it doesn't meet ICAO requirements (as FAA student, sport and recreational licenses do), then when you are flying an aircraft registered in that same ICAO member state, that state's licensing rules apply and all other states must accept it as valid regardless of their own rules.

So, in your example, if the FAA says your aircraft is a "glider" and you have a private glider license, then you're legal in Canada even if, for instance, Transport Canada thinks your aircraft is an "airplane" and would require an airplane license for their own pilots to fly it.