What kind of fork could i fit on my bike
I wouldn't consider anything over 140mm. beyond that, you're really messing up your geometry and endangering your frame.

Keep in mind that your frame's limits are really determined by how much you weigh. If you're 140 pounds, the frame will take much bigger jumps than if you're 240.

You say that the rear shock is really stiff. It seems there is an imbalance in stiffness between the front and rear suspensions. If you don't have the preload on your Suntour cranked all the way up, you should try doing so before upgrading.