The z6 uses a different intake than the z8. Not sure how it sucked in sand. The foam filter works decent unless it was put in wrong. I might do a compression test before I'd look at a rebuild. Might just be a nicked up valve face or seat and that's doable. The is also cutting the oil filter open and checking with a flashlight for metal flakes in the pleats. No flakes and your bearings should be ok. Compression test will tell you if it's just the valves or rings (and maybe the valves too). A top end is not a big deal, just takes time.