Generally there are quite a few changes from ETOPS to non ETOPS aircraft however they very from aircraft types. Generally you need a additional independent electrical source (usually a HMG) that will function with either engine shut down. A APU certified to start and supply electrical power at all altitudes. A cargo fire suppression system with more agent capacity and a metering system. Avionics changes are often needed and a dual HF radio installation. There are numerous other changes that may be required.
There are as mentioned quite different maintenance requirements including how engine performance is tracked and monitored.