I got a VW powered Nipper up to 10k, it has no mixture but still made it. I have flown my Rotax powered machine up to 10k several times, and this also has no mixture control. Interesting that the LAA test now calls for a 5 min climb. I will be at around 7.5k at the end of 5 min so I will have to fly a x country to avoid CAS.
Do a cruse power decent and take your time. Touring Scotland or the Alps it is often necessary to get up to 10k as it is smooth even if the wind is not so favorable. You will wonder what all the fuss was about when you get down.
Rod1