The aircraft is no longer certified. Owner maintenance is kind of like an experimental. Once an aircraft becomes OM it can never goback to certified nor does the FAA allow them to fly into the USA. It’s a class for older, simple to maintain aircraft that are no longer used much commercially. The 150M is not eligible for that reason, all other 150’s are. More info here.
https://copanational.org/en/owner-maintenance/Can be a good option but some say it limits resale value. I would think seriously about it if I had a plane of lesser value but my 150 seems to be appreciating nicely!