Ack!! Bummer...you got one of the newer cases....
But that's good in a way...cause you have twin-ring version...all the new cases I've seen have twin ring pistons.

The flange isn't too hard to grind off. I think it's pretty soft metal...But it will make a mess. Sucks that the company went this way...maybe to thwart our conversion attempts?
Or 4 bolts was too expensive to keep using to hold the flange on....

Try the hacksaw...it may cut easy....you may be able to have local machinist "work" it off for really cheap if you tell em what your doing with your old weedeater...local guy here laughs at me and makes good conversation...

Can't tell if that's the long or short version....need a side pic.....I like the long shaft better cause I know my prop is riding on the actual crankshaft instead of threaded hub.
But I have two short shafts too.....work just fine..

Make sure you specify when you order parts which shaft you have.

Good Luck...start grinding!!
CrazyHerb