I’m in the market for a 9A, and briefly considered the same or similar plane. But based on discussion on other forums with someone who actually had one with an O-290 and later had to swap out, the O-320/360 mount 2” closer to firewall necessitating a different mount than original AND a shortened cowling. I’m not sure what mount was used originally. So definitely more work than just swapping out the motor according to someone who did it. The well documented lack of parts discussed across lots of forums made me concerned about being stuck with no alternative but to replace the motor when something fails. If I were younger, I might consider what could end up being a project plane. It’s definitely attractive, and if your circumstances are different and you are ok with a potential project someday, I’d try to negotiate on it. As previously noted, it would give the opportunity to swap out the nose gear.
Over the years there is occasional discussion regarding using non-standard motors and their impact on resale. I recently watched a very clean O-235 equipped 9A sit for sale for quite a while and that motor has ready parts availability. I don’t know what they eventually got for it but they were asking $90k and it had a digital panel. The planes with Subies really sit and you see prices drop.
Just the opinion of someone studying Barnstormers and Trade-a-plane multiple times a day!