TBH I doubt Bryer actually put it together himself. I think maybe that was one theory the RCMP mentioned and somehow it got misinterpreted by either Alan's lawyer or the reporter as "that is what happened."
Reasons why I think it's unlikely:
a) The report said they don't know where it came from, so how would they know if he put it together?
b) While I can imagine Bryer being into the concept of building his own gun, I can't imagine him actually succeeding at it. These guys failed at life in general.
c) The older SKS rifles are often made of different parts and can be obtained very easily for cheap (about $200).
d) Private sellers often sell guns without following proper procedure. It's even possible Kam bought the gun since he had a PAL -- why risk a weapons charge for Bryer if Kam could get it legally? -- and the seller just never bothered to deal with the paperwork. (Although people who know more about Canadian gun laws, your insight is much appreciated on this topic!)