If it was mine I would add a couple of tubes across the rear rails, just in front of the rear wheel. It'll stiffen it up nicely and give something to mount the mudguard to.
And I think you should add some gussets between the rear engine cradle and the rear top rails as well.
yes they are coming im trying to get wider wheels for it cheaply so been leaving mudgaurd for now and so will do cross pieces when got wheels....
i am going to gusset it as well again not sure how long to make gussets i have some long and short cut and still thinking want to get seat thought of first
My friend is doing a Guzzi hard tail but is going to put a chair on the side as he only has one and nearly a half legs. He does ride a quad at the moment.