Went through exactly this a few months back with my 200 EFI Suzi, so I can understand your frustration.
Prompted lots of discussion on here around the use of JB Weld etc.
I was lucky, I found a used bottom end at a very good price, however I was going to get a welder to have another go.
My experiance with jobs like this and welders, is that you get what you pay for, and welders that can weld thin wall cast alloy are few and far between.
I'd advise taking a half day off and phoning every outboard dealer you can get the number for. There must be one somewhere, even if you need to buy a complete but tatty engine.
RIBBED For extra pleasure.
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