Shame about the 100 planker, it was a bit of an icon. Still, that whole downhill section from rooters onwards is insanely good fun. Not too bothered about the bridge on distillery, that was definitely getting a bit crusty! Would be great to know what structures are permitted, I'm more than happy to donate for wood or other materials.
I hope you've got deep pockets. I was there with the DELWP land managers when the Distillery and 100 planker bridges were looked at. Since this govt dept has no guidelines yet for MTB specific bridges they have to follow design standards for walking track bridges. Anything with a drop to the ground from deck level greater than 1 m requires handrails (not sure how high above the deck these need to be). Likely minimum width 600 mm. All man-made structures (not applicable to dirt ramps and logs/rocks lying on the ground) must have a structural engineer sign them off and then have periodic inspection and probably maintenance as well. I'm sure there are others with more experience in trackbuilding who can give a more authoritative answer.
For replacement of the 100 planker the ranger's estimate was in the ballpark of $20k - $30k.
In our opinion, if the dept could find that kind of $, that would be better spent elsewhere. e.g. it would buy lots of decent-sized rocks that we could use to armour and/or realign sections of Wombat #1 that are eroding. We aren't as geologically blessed with readily at-hand rock as other MTB areas.
Since motorbikers have found and started using the Rooters downhill/Gap jump and 100 planker tracks in the last few years they're become very difficult to displace and they're going to continue to rip these tracks up. These aren't formalised MTB tracks so we can't enforce any motorbike exclusions. Rooters downhill/Gap jump is still OK after a crew of 4 spent a few hours filling unavoidable ruts with rocks and dirt - not something we're keen to do too often while the motos keep using it. Enjoy it while it's still a clean run. With the 100 planker gone we'll probably stop riding and maintaining that track and leave it to become an ugly erosion gully.
To answer your other question, within reason, any structures on the existing formalised tracks that are made of natural logs/rocks/dirt that aren't significantly elevated are OK.
No new tracks can be made without permission, fines of around $5k to those who are caught doing so were mentioned. So we (or you) can't just go out making new stuff without planning and land manager's permission.