If there is no service history provided, I would go for a complete service, including;
Aux belt, 3 spark plugs, air filter located near oil tank assembly, fuel pump diaphram kit, fuel filter located in the FSM, gearbox oil, impeller and related gaskets, impeller hosing, wear plate, and thermostat.
The list above may seem abit OTT but that is everything I would advise doing for piece of mind. That said, if you do know the service history, you could get away with far less ie; plugs, fuel filter and gearbox oil.
Its all pretty easy to do. The Opti's are one of my favourite engines to work on.