250-500cc WR motor ser numbers...

Was talking to a fellow online here in Canada that works at a Salvage yard. Says he has a few water cooled Husky motors there. I'm looking to do a 6 speed tranny conversion. Problem is, he doesn't know what year and/or model any of them are. I told him to look for lighting coils and to go through the gears to see if they're six speeds, but that still leaves the question on what year they are. Is there a way to check from the motors serial numbers? If so, can u guys point me in the right direction? If he has more than one that are for a WR 250/300/360, I'll let u guys know as I'm sure I'm not the only one itching to do this mod. Any help is greatly appreciated. Thanx.