Good suggestions, guys. Thanks.

Sadly welding is not an option, as I've never done it and don't know anyone who can!

I think I'll try Tanto's suggestion- my neighbour will lend me a scissor jack and axle stands, then I'll put 2x4 wood under the engine to support that. Getting the carbs on can put a lot of force through the bike so it's gotta be solid before I try it!