So I have been emailing with Hagon directly over the past couple of days (took that long due to being half way around the globe) and found out some info. First they had me measure the distance between the shock mounts while the tire was just touching the battery mount tray, which came out to 285mm or 11.25". They also said that ideally I should have at least 90mm travel between fully compressed and fully extended, putting me somewhere in the 375-390mm extended length.
Their response:
These dimensions do bring up some issues and the first one is the length!
390mm is trail bike territory and this also means longer travel (120mm) but also spring rate and how they ride is not ideal on this sort of bike.
The next problem up is the shock mount on the swing arm.
Normally our shocks can be supplied on 270mm unit’s right through to 370mm units and in 10 steps.
On this generation Honda machine the bush is often in the swing arm which of course means the shock has a fork fitting 20mm wide on the bottom… this is hitch 2!
These fork bottom shocks are only made in 290-310-335-361mm lengths.
This means anything near 390mm is going to be fabricated from scratch and consequently dearer!
If… you were to change the lower mount so the machine can except a more common eye top, eye lower type of shock it will create a lot more freedom and we could supply a set of shocks with adjustable damping for a cost of £226.50inc a pair.
To scratch build from the ground up a fork bottom 390mm shock would be closer to £325.00 the pair.
However, 390mm is a very long shock on any road going chassis, just have a check that when the engine is in the frame, wheels in and sprocket fitted, that the drive chain has clearance over the swing arm pivot tube.
When you go from a standard shock around 335mm right out to 370mm plus, that clearance closes up pretty quick and its worth calculating before you make your shock order!
Looks like I am back to the drawing board. I will check out some of the other suggestions you guys made, thanks.