Man...oh man, there is a lot here. Other than things that apply directly to the Cessna 150 (ie, the elevator bracket SB, flap tracks/rollers, seat tracks, etc) there really are not a lot of problems on the O-320 conversion.
1) Engine: Normally an O-320 E2D that powered most of the 68 to mid 70's 172's. Very bullet proof but not without AD's. Oil pump gear AD may apply, checking the end of the Crank for rust and greasing AD may apply. You will want to check both the main screen (unless there is a filter, then cut the filter) and the finger screen at the bottom of the engine (oil inlet). Look at what prop is on the bird as there is a whole book that can be written about props and their variations, but it seems Wayne W. has the perfect combo prop pitch.
2) Have the exhaust system checked. There is only one way to fix the exhaust, and that is to find a shop that will build an entirely new on using a couple of pieces from your old one. Baffles, welds, etc.
3) Where is the battery...some are in the nose with lead weight in the tail, others (like mine) relocate the battery to the tail.
4) Long Range tanks....if it doesn't have LR tanks, move on to the next one...you WILL regret this decision if not. They are not a cheap install and you are not going very far at 8.5-9 GPH without them.
5) Paperwork/Paperwork/Paperwork. Again as I said before. Don't get "buyitis" and skip the paperwork. Something as simple as replacing the electric boost pump (yes...electric boost pump...what on a Cessna?) may come to a grinding halt since you need the drawings to prove to some AP's that the "model number" of the new "automotive Facet pumps" are acceptable since they don't build Aviation certified ones. Don't ask how I know this.
6) Quality of the cowling modifications: Biggy here potentially. The STC was actually modified at some point on the Avcon when the cowling began rubbing holes in the number 1 valve cover. A "WAD" of duct tape was the modification (oh for goodness sakes) to prevent the chafing. Again...don't ask how I know this one. A bit of decent metal work fixes this problem.
Finally, be aware that the airplane has a forward CG compared to it's O200 parent. So...keeping mindful of the nosewheel is very important, but not difficult. It made the transition to my Cardinal a piece of Cake. But, the unwary can slam the nose down if not minding their P's and Q's on landings.
If there is STOL or ACT "Long range" tanks installed, again, make sure the documents and weight and balance in order or walk, as you may buy a technically unairworthy bird.
Oh...and when you go to have a checkout or flight review,,, I have run into two instructors (one 150/150 owner friends instructor and the instructor for the guy I sold mine too) who won't fly it over GW. Simple answer here...it is a 150 after all, fuel it down for the check ride, then control your fuel consumption with the little black knob your hand attaches to during take-off and landing.
My O-320 when flying to Clinton with two stock 150's burned a paltry 5.5 GPH hauling a "tad" more cargo than the stock birds. I actually skipped refueling at some stops. It is a great bird to fly around with windows open at 2000 to 2100 RPM (unless of course yours has the prop engine combo that restricts "prolonged operation" between the 1800 and 2200 RPM mark-if my memory is correct).
