That’s a bit of a weird request, given that there is no such thing as an “Discourse mobile app”, as in the end it loads the actual Discourse web interface.
Sounds like what you really want is your forum to migrate to Discourse and provide the Discourse experience to everyone.
Nowadays a migration like that shouldn’t be too expensive to hire someone for if it is a small community. Did they try to hire someone for it on Marketplace ? Maybe the community can chip in and hire someone for a one time seamless migration ?