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CB550F2 Cracked rear engine mount

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mtrhead:
Hi folks,
Quite embarrassed to say my 79 CB550F2 rebuild fell by the wayside since I first acquired it in a seized state and posted about it almost 10 years ago. Life, a CB350K4 restoration and other stuff got in the way but I'm determined now to get it done. Just prepping the frame and after having had the top end sorted I'm now turning my attention to a problem I hope isn't going to defeat me!
I stupidly managed to crack the left hand rear engine mount casting after an attempt to pull the engine out on my own. I now have two problems! The original engine mount bolt is completed corroded in what is left of the engine mount casting and I can't get it out. Secondly, assuming I get the bolt out I've then (presumably) got to get the broken bit of the casting welded back on.
Just picking the collective wisdom here:

1) I searched the forum but just wondering, has anyone else hit this problem?  I've pretty much mullered what is left of the bolt trying to drill it, angle grind the side of the bolt but its defeated me so far. I'm wondering if I can perhaps stick weld a nut on the remnants of the bolt but I'm wary, I have a cheap and nasty stick welder but I've never used the welder in anger and am worried about ruining the casting (further!)
2) I'm pretty sure the bottom end of the engine is good. I don't really want to go hunting for a replacement bottom casting as I suspect I'll just hit more issues with this approach. However I wonder if its actually going to be less pain in the long run. Any thoughts, what would you do?

If anyone has any recommendation for a good welder in the Bucks area that would be good. My (very local) engineering shop weren't very complimentary about welding bits back onto a Honda engine when I dragged it down there for a viewing  :)

Seabeowner:
Is that the upper or lower mounting?
Not that its any use I had an awful job removing the upper bolt on me F and had to saw it off to get the engine out of the frame. Tried whacking t, soaking, heating to get the remainder out. Had to drill out one side before it shifted.

mtrhead:
Thanks Seabeowner, its at least good to know others have struggled here before me. Its the lower mount!  Clearance is a bit restricted, have burnt out a couple of flex drives and blunted multiple hss bits already trying to drill the remains of the bolt out.

Bryanj:
Welding old corroded alloy is not easy, you probably dont want to hear it but replacement cases is easiest way to go

K2-K6:
On various recent "projects" I've experienced that corrosion bond of steel bolt in aluminium, oh how very secure that can be, far beyond how you could imagine it.

Various tactics, Asda maintenance spray to soak is most effective I've found. Takes time to get any discernable movement at all, ultimately using an SDS hammer drill with rotary stop to just keep rattling it. You'll likely need ear defenders too  :) they don't seem like they are going to move, but more and more corrosion dust slowly comes out until they eventually yield.

Without pictures it's hard to give orientation opinion, but if the "outer" flat surface is available to place on a vice or similar, then approach from broken end, possibly  ?

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