5 bars is hot..if you keep getting air ,look at the recovery bottle while hot and running if you see air bubbles, its a leaking head gasket .been thru it
Can't say on the whole rig but my experience with HISUN engines has not been pleasurable to say the least. Makes you wonder why they want to dump it with only 100 miles. Have you actually Seen it or just an ad? Beware of scams. See clear title before you buy.
Remove passenger seat. It's a typical m/c type dipstick on the side of engine. If you need to add or change oil, be certain it's WET CLUTCH approved engine oil. Full synthetic auto type oil will fry you clutch quickly.