If your ISP identifies the OpenVPN traffic just by filtering the traffic to port 1194, you can just easily change the port in the configuration web page of IPFire.
If instead the ISP is doing deep packet inspection, as you said you need to encapsulate the traffic inside another protocol e.g. the HTTPS protocol. For this purpose you need to install another server (a sort of reverse proxy) running in front of the OpenVPN that takes the packets coming from the OpenVPN server and encapsulates them in HTTPS packets. Therefore you also need another proxy server that takes those packets and correctly de-encapsulate and forward them to the OpenVPN instance on the other side of the tunnel.
This is true for client-server communication (roadwarrior) or server to server (net-to-net) communication.
Here is a tutorial on how to do that in ubuntu using obfs4proxy as a reverse proxy server. This software is not available as a plug-in in IPFire, therefore either you package it or you use another machine in your lan to do the reverse proxy job running obfs4proxy. Another possibility is to use a reverse proxy available as a plug-in in IPFire like nginx, a quick google search gave me this tutorial, but I have no idea if this is accurate.