Hi Rob,
You could measure the two flat sides of the nut to get the size right for a socket or buy a socket set.
You can bend a bit of metal to fit on both sides then use a ruler or cut out some cardboard and measure that.
Norm had this problem and instead of spending money and time on a rebuild just put a Victa 2 stroke de compressor in.
If you replace the camshaft to fix the decomp problem there is no guarantee it won't just break again.
Cheers
Max.
