It actually is semi official. I used to help build when I lived on the lake. This guy Darren went to Jefferson township several years ago and got a proposal approved to clean up the property with rights to build “recreational” trails between liffy, prospect park, and all the way out to mason street. The town may not know there are like 5’ drops and berms being out in on the island but…eh?. The trails are sort of enduro style with some tech climbs up the mountain and pretty fast downhills back to the island with plenty of rollers, drops, etc. it can get pretty confusing if you don’t know the route because there are still a bunch of kids making their own illegal moto stuff back there.
Here’s their Instagram
https://instagram.com/lake_hopatcong_trails?igshid=YmMyMTA2M2Y=
And the Strava club - they usually post group rides in here and it’s he best way to learn the system.
Try to check out a group ride if you can. They are great guys to ride with.