OK, so an update!.
I have managed to find a very little used from new Trax rear shock (correct for the year that the older MX suspension used) and have also been lucky enough to have it custom made to the stock 490mm length the MX uses.
Had it valved and sprung and a bladder kit too, all new seals , piston , bushes , piston ring, bladder, bump stop & a new spring all rebuilt as-new. After looking at @Philip
chart and the work and time he had put into trying to find the best set up, and taking into account his ability and weight we went with a 66-260 spring in the end as i am 208.9lbs now (been hitting the Gym a lot - more muscle!) so with rider kit should be about right, hopefully!.
Even had custom machined spring collars!, as the Trax shocks use a non standard size spring diameter which is obsolete and now I have fitted a standard size WP linkage spring of 63mm I/D so its more easy if I need make further changes. Many thanks to Julian for his time and help!.
Fitted it today, I was aware that to fit the Trax shock onto the bike, the mounting area on the MX frame needed some grinding to give it the room it needs, mmm....a lot of very careful grinding!. But it's all done now and very happy with the fitment and finish too.
Thankfully i did not have to worry about the clearance of the shock with the rear subframe as there was plenty of room. So, for now all done on the rear and I will have the bike all back together by the weekend. No battery in the bike atm and man does that bike bounce back sharp if you push the seat down hard - i guess that 5 stone battery really helps!.
I really hope the Alta is going to be so different to ride when i get it all sorted out and go to test!, am very excited as never had any suspension set up for myself, ever. I know its all trial and error, especially with the Alta and it's limited knowledge for tuners, well over here anyway, and it being the older Alta too. I just hope it's all going to be worth it.
Will have more to post up soon - on the front forks i have gone with. Thanks for looking if you have, can't let this site be taken over by the Varg - Alta was first!. #76.