What modifications need to be made on an engine that normally uses AVGAS to utilize it with MOGAS?
For any certificated aircraft engine, the type certificate will specify the required fuel. If the TC specifies AVGAS, using MOGAS will require an STC for that engine.
Developing a MOGAS STC for an engine is a complicated and expensive effort, so you're realistically limited to purchasing the rights to an existing STC. If there isn't an STC for your engine, you're likely out of luck.
The two most common STCs for GA piston engines are available from EAA and Petersen Aviation. Check their web sites for a list of engines covered by their STC.
Once you purchase the STC, it will include the installation instructions outlining the necessary modifications for your engine. In many cases, it's nothing more than placards and a flight manual supplement.