Oh man, you really stepped in it this time! This will get all kinds of responses, on both sides of the aisle. A properly built 12 will do just about anything an 18 will, but its a lot of work to properly build a 12. A well built 12 will have a 150/160 on a short mount which requires all new cowling and re-wiring/re-plumbing through the left side of the firewall to accommodate a swinging mount like the 18 has. You will also have to buy an 18 exhaust system and airbox. Now its on to the landing gear, which gets swapped out for 18 legs and external bungees with custom length shock struts. In order to do this you'll have to weld in an external cabane structure. While you're at it, get 3" extended gear to help with the 12's lower angle of incidence (a lot of hay has been made over the incidence issue, but with extended legs and 31" Bushwheels its a non-issue). O.K. - take a breath... Now you have to open up the wings and install flaps using either 18 or 14 hardware (14 is really the way to go here, the flaps are slightly longer). Of course, you'll have to install the cabling into the fuselage and the flap handle assembly. Finally its back to the tail where you'll need to swap out the elevator & horiz. stab. assembly for that of an 18, which is aerodynamically counterbalanced. A couple of other niceties are an under seat battery and wing root sight tubes for the fuel. You're also going to want a swing-up (seaplane) door. I've probably missed a couple things, but those are the biggies. Obviously were talking about a complete teardown and rebuild here, which is the perfect time to do it. In fact, if you really want a tricked out 12 and have the budget, I recommend buying a tired airplane and doing it in one big swoop. All else being equal, the 12 will probably end up 30 or 40 pounds heavier than the 18 due to the extra steel in the fuselage-can't be helped. When you get a 12 built to this level, the difference in what it can do vs. an 18 will lie more with the operator than the machine. I have a 12 with all the above except the short mount, and that happens next month when it goes in for rebuild/recover/paint. Its a highly capable aircraft and I LOVE the interior volume (extended baggage is a must). In order to build a modded 12 a lot of parts have to be fabbed, which could otherwise be bought if it were for an 18. Now, all that having been said, figure out your budget and go from there. A common 12 upgrade(s) is a 150 and flaps, and this is a great start. There are plenty of them out there and such an aircraft can be had for way less than a stock 18/150 and you'll have a fun, affordable STOL aircraft that will take you a lot of places. On the other side of that coin, the stock 18/150 will be the superior performer. A well modded 12 is definitely a labor of love. And money.