I Have a wing safety 1712, and I'm after a very light two stage trigger. The 1712 is fitted with a 5109 trigger and was wondering if anyone would know if a 5098 Trigger will fit?

No it won't fit.

I Have already replaced the main spring with j p Products spring kit , and it is now just under a pound, Is there anything else can I do to make lighter?

The trigger that came in your rifle is designed to be used as a two stage trigger, you can set the first stage to about 1 1/2 lbs. and the second stage to break between 9 and 10 ounces. It can also be adjusted to a single stage, but probably not less than a pound.
By the way, for Hunter class the trigger most be able to pick up a 2 lbs. weight before it breaks.
Evelio.