I'm no expert but I'd be seeing how it runs on petrol to make sure the fuel system has been maintained, you're supposed to run petrol once a week or so which also helps the engine (so I'm told, I didn't think unleaded had lubrication properties) so if its runs rough, maybe that side of things has been neglected.
Other than that take it for a drive and see if it idles smoothly and doesn't hesitate under acceleration (on gas).
Maybe also look at the airbox, expect the occasional backfire so see if the airbox is cracked, recently replaced or has one of many homemade "plug" mods to prevent damage to more expensive parts.
Clacks