I suspect that originally you had an AQ271 (or AQ260) engine with a 280 outdrive.
The 280 outdrive is indestructible, but the technology has moved on and the newer drives more efficient and give better performance. As we discussed on the phone, you shouldn't try to use your tilt system to trim the boat as it wasn't designed to handle the stress. Eventually the vise rod will bend and the tilt won't work. The trim is set by the trim bar on the exterior of the transom shield and the idea is that you put the bar in one of the five holes which holds the drive at a certain angle.
If you wanted to you could swap out the lower unit of the 280 drive with a 290 dual prop lower drive. It takes about 15 minutes to do the swap and then you would have a dual prop. It still doesn't have a trim, but you will get better performance and some fuel savings. A new lower unit is about $4,200, but I bet you could find a good used one if you look around. You want a ratio of 1.95:1, but for about $2,500 we could change the ratio if you find a good inexpensive one out in the world. Sometimes you can find a complete unit and just use the lower unit and sell the upper and intermediate units to help offset a cost. A new aluminum prop set is about $650 dollars. If you look you can use a 290DP or a DP-A or a DP-B on your existing set up.
The other option is to swap out the entire transom shield and 280 outdrive with a new SX or DPS outdrive and SX transom shield. This would mean that you have a drive that is newer and has a fully hydraulic trim and tilt system. You would get better performance from either of them and the dual prop would give you better control at lower speeds, better fuel economy and about a 15% increase at the top end. You would want a 1.6 or a 1.66 ratio for the SX drive and a 1.95:1 for the DPS drive.
Volvo Penta makes a nice adapter plate that will allow you to put on a new transom shield without having to do any work on the transom (the hole). The adapter plate costs about $700 and while I doubt that you will find a used one, you should be able to find either a good used SX or DPS drive somewhere. Again if you find a drive that is good and inexpensive, we can always change the ratio (most times) for about $3000.
A new transom shield is about $3,500 and the SX transom shield is used for both the SX single prop and the DP-S dual prop. Used transom shields are fairly common and sometimes you can get pretty inexpensive ones and then spend a little bit fixing them up.
A new SX drive is about $7,500 and a new DPS drive is about $9,500, but used ones are out there. We often find good deals in complete boats that have had something terrible happen to them. You can strip off the drive and transom shield and then scrap the boat.
If you find some used parts that sound interesting, please feel free to contact me about them and I can give you an idea on what they might be worth or what it would take to fix them up.
Please feel free to call me if you need anything or would like to discuss this further.