Jip are you sure it is running on 4 cylinders, sounds to me like the engine needs a compression test and tune up If its shaking so much that you can't keep air cleaners on it.
your noise will be either a clutch in backwards,a thrust bearing or a sprigot bearing dry.
easyest test for your brake booster, it should be able to apply 1 to 2 aplications once the engine is switched off.
Make sure both venturi,s are the same size, also be carefull not to swap venturi tubes with a standard carby as singles will be slightly larger and you will develop a flat spot.
let me know if you get stuck with panel shapes as i may be able to help.
To fix heat worpage use a Shrinking Disc , it fits on an angegrinder and heats the high spots then you quench them in cold water. even a beginner can get great results.
check out the link below i have also seen them on ebay.
http://www.daggertools.com/m5_view_item.html?m5:item=DT-SD2