Thanks the Arena info was helpful. Section 11 I had read and compliance to two of the three would be OK as both ideas would pass the 20% rule. Not kinetic mass type spinner, (essentially it's about big impacts) but a cutting shredding weapon. Which I realise brings many challenges but, I just like the idea. I think it could work in lower classes where more plastic and aluminium.

With luck my esc arrives today. Bit worried about size, after comments on here. But it's definitely the right model shown in the Haynes Manual. If I blow it then I am going to be in trouble, with Christmas coming up going to hard to afford replacement parts.