Just a thought I had, our local tractor dealer has a lot of different size quad bikes and I went there to get some fuses the other day and took a quick look at their suspension systems. A lot of the shocks look around the the size of herald shocks but with adjustable springs as well. This may be worth looking into, a spring rate of 200lbs is what was fitted to to the early Spitfires and they are about the same weight as a Marlin, some of the quad bikes look to be about the same weight also.

Where you are based this could be an option.

Cheers

Phil