I was doing my first gearbox oil change on my recently bought 1999 CRM250AR today, I dont know how but i couldve accidentally tightened instead of loosened and managed to break off a part of the sump where the drain plug goes in.
I have no idea what to do and will be very grateful for any information/help/guidance as to what I should do.

Here is a link to photos of what has happened
http://postimg.org/gallery/16opus9xq/

I hate to say it but essentially it looks like you have 2 options.

Replace the right hand engine casing if you can find a 2nd hand one.

Or if you still have the part that snapped off have an engineering outfit or a mate who can do a decent job of alloy welding reattach it it and try to re tap/repair the thread.

Neither option is particularly simple or easy but from what it looks like i think that's your lot :-/ reattaching the snapped off part means you don't have the hassle of a strip down and engine split meaning new gaskets and such however depending upon how clean the break is it might be a real pain to line it all up so as to get a nice clean straight thread back in there.

For any sort of longevity about the repair it needs to be the case splitting option. If the engine is stripped, initially see if that can be repaired using the snapped of piece.
If you do strip the motor,it's almost worth checking replacing other wear items, like the mains, seals.
Personally, I always use a ratchet for undoing that particular nut, that way you can ensure the ratchets set to undo, before exerting any force on the nut.
Mike

ahhh thats a total shitter!!! Welding wont work as it will warp the case. only option involves new case(s) and full engine strip!!
I have a spare engine but all the time I own an AR I am keeping it for this exact reason. Parts are getting harder and harder to source unfortunately.

Splitting the case would be essential to repair it properly. You will have to make sure the gasket face is flat once the broken bit is joined back in.

Welding might be a last resort if nothing else works.

I had some "welding rods" for aluinium but they were more like a pig alloy that soldered the aluminium together. That way they didn't require as much heat as welding (which should reduce the chance of warping) but you did need to get a lot of heat into the area as the rest of the aluminium wicks the heat away from the join.

Another option might be JB-Weld, just make sure all the oil is cleaned from the bonding area.

Did you end up fixing this? I can vouch for jb weld.