Paul, I think you are out of luck with the shocks/springs. There are Kymco upgrades available with gas cartridge shocks, but I doubt it will make the ride "softer". I think they are on the Kymco Site under Accessories, and about $300 the pair... Since I don't have them I can't say more how they work. Also, don't forget the G/V is a light weight bike compared to your K75, so it will give up on ride comfort. FYI: I just rented a Daelim S300 scooter in Europe for a few days, newest model, and it rode not much better than the G/V. So, I think ride comfort is weight and specific suspension design related. The G/V is not great in that department.
Changing rollers and belt: Make sure you have an impact wrench handy with (I think) a 17mm nut. I had just thick leather gloves and held the Pulley and the impact wrench opened the nut up. I also tightened the nut with the impact wrench and put a "serviceable Loctite" in the thread/nut. 100% correct is of course according to torque specs, but then you need a special tool to hold the pulley to do the specs. It worked for me on three G/V's that I serviced for myself and friends Just watch the nut when it stops moving. You don't want to have the pulley assembly come loose so use the blue Loctite or equivalent.
Make sure you hold the assembly from the back with your fingers before pulling it off or all the weights fall wild out, which is not really a big problem because you replace them anyway. There is also a specific way how the weights are going in; it is on the description. Has something to do how the rubber rollers are pressed on and turning direction. Just as precautions.
As to the rattle it may be the exhaust (check also if all heat shield bolts are still on it; the get notoriously lost), or it can be the slap of the belt on the bottom if stretched.
