[TrialsRider]

To spring the rear heavier, you can either replace the spring with a heavier rated spring or change the angle on the shock so it's mounted more vertical. If you remove the spring from the shock body and mount only the shock body on the frame, you can determine if the mounting angle is going to provide the correct amount of wheel travel, if you already did that and the shock needs to be mounted as it is now, then a heavier spring is the better option.
... springs are not cheap, you are looking at 100$+ and it would be wise to determine the spring rate of what you have now so you can shop for something heavier duty.

 

[TrialsRider]

Preload can only do so much and then eventually you need to go to a heavier spring, nothing wrong with wood sliders for a chain, that's a perfectly good bush fix, chains going to ride on the centre rollers anyway. The steep swingarm can make the rear launch when you put the power to the chain, it's going to compress your rear suspension lots.
Test rides will tell you what you need to do