man thats a bummer, can you get a good photo of it Dicko?

hard to say from that pic, its not really not clear enough but helicoil may do the job, depends how much has chipped away??

You need a point of sealing.
As in, a flat surface for a washer to seat against, or a machined lip for an oring to seat into, etc.
It looks as if you have disturbed that dynamic.(Not exactly conclusive from the pic, though)
But if actually is damaged in that manner, it is a little hard to envisage what a helicoil would achieve as far as sealing goes. I imagine it would leak like a sieve.
Possibly convert to a large tapered plug(as per early primary, but bigger)? A weld repair could be quite involved.

yeah its either tapered plug where the threads do the sealing or a flat surface where the gasket or o-ring does the sealing,

as I said the pic really isn't of enough quality to suggest a decent repair over the net.

I must say im not a fan of tapered plugs into alloy housings....

A standard casting repair would be to bore and thread the hole and make a plug to fill the hole and seal it with loctite or thread tape and sealant, the plug would have a hole and a plug to use for future draining, helicoils are crap period as they are no more than a spring and end up coming out with the plug later on when you want to drain your oil, tappex inserts or solid inserts custom made for each job are the way to go.My 4 speed has the same repair i did 16 years ago and still going strong. If you feel you are a bit heavy handed buy and use a torque wrench.
want to repair a leak? ask a shovel owner.

i have seen tapered sump plugs for when this kinda thing happens, they also cut their own thread, and have another smaller o'ring sealed sump plug in the centre of them, screw them in with loctite or jb weld and its as good as new. http://www.cgenterprises.com/drain_plugs_oversize_repair.htm

If it were me and didnt have alot of dollars to get this fixed "right" i'd defently be trying this fix first. At the end of the day the only other way i see to fix this is to remove the motor, and use a wel on bung with new thread and sealing serface....

Keep us updated.

Regards Andrew