Is it possible to replace this quick release part on a Motiv Rockpoint?
Quick releases are standard parts. You can replace the whole unit with a new one. Normally you buy them in pairs, front and rear together.

All you need to know is whether it is for a mountain bike or a road bike since the size of the rear one varies due to different rear axle lengths.

What is more worrying is the fact that the nut came off first place. This can only happen if the QR isn't properly fitted which is a potentially dangerous and even lethal situation. Check this video: