Jimmbbo I think you are on the right track here and I have used M4 nylon bolts on smaller twostrokes (up to .40 size) without problems but you have to be careful and inspect them regularly. One compromise worth considering is to use brass bolts - an old idea I got from white haired CL guys flying on tarmac. That is what I use now on both 2C and 4C engines without problems at least up to .65 size.
Steel bolts is an absolute NO-NO..[:'(]... in my ships..................Cheers/Harald