I live in Sarasota and have worked as an environmental consultant down here more than 10 years doing wetland permitting for local developments. I haven't ridden any of the North Port trails but am aware of issues the City has through my work in and for the City, and from tracking local proposed law changes. The good news is the City's so dysfunctional and broke, they're probably not going to do anything soon. The bad is that these trails are on the City's radar and they're trying to get rid of them.
Myakkahatchee Creek is part of the City's water supply source and of concern from a flooding perspective. City stormwater manager is a former SWFWMD compliance engineer. Proposed changes to the City's comprehensive plan specifically prohibit trails in the 100-yr flood plain of the creek out of concern for flood protection (stuff might get caught up in the bridges the club's built and block water flow), protection of water supply and general protection of natural resources. From discussions I've heard, the City has paid consultants to walk the creek and probably know where the built stuff is. If the prohibition language gets in the Comp Plan, the trails will be illegal. If or when that happens it wouldn't suprise me for the built stuff to get torn down by the City.
I can't imagine properly built single-track would be an issue. But because a bunch of structures have been built on City land without approvals/permission and these create a safety liability, potential flooding concerns, and potentially impact other natural things in the City's mindset - I suspect the battle is going to be an uphill one. Remember you're dealing with the City of North Port, who is in the middle of taking somebody's house for 10's of thousands of dollars in Code Enforcement fines because some lady didn't buy a $30 permit to put an awning tent over her lawn furniture in her back yard. Good Luck.