How resolve blade issues?

I have this saw (Challenge 1500w) I use many blades i have a lot with 30mm bore the challenge dictates 20mm bore the saw is constructed in such a way the reduction system ring does not work.To resolve this it appears a alternative bore boss to locate the blade is required is there anything that will fit perhaps this would be from a different machine alltogether or will i have to make one ?

Markie
December 2011