[2015 Z Force 800 53/Trail EPS]

A company that is and has been a sponsor on my other powersports sites remanufactures all kinds of engines-mostly powersports related. They do it right and are very trusted. They hold the same values as we did when I was with Honda so many years ago. Might give them a call and see what they say about yours. You can also have modifications done to it like BBK, cams...ect at the time and it comes with a great warranty. Great people.  Here's their site:

https://www.nflowmotorsports.com/

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.