The 912 is not a car that is well suited to the search for more power. With a skilled driver it can be fairly quick, and mods that aim to enhance it's strengths can be effective, but power? Power will tend to be among the costlier mods, and least effective. If you are looking for a few horsepower, looking into an early factory sports exhaust along with adjusting the carbs will probably do that. Beyond that? Rebuilding the engine with different pistons and cylinders and moving to a more aggressive cam (though you will spend alot compared to the gains you will get and may Vag 15.7 compromise the drivability of the car to make a meaningful gain); or looking at a different engine (which will bring a whole new set of challenges since it will change the weight distribution of the car: either with a 911 engine making the back heavier like a 911, or many of the other swaps with their higher centers of gravity that will make the car very top heavy in turns and destroy it's handling - as well as require suspension upgrades and brake upgrades to handle the greater weight and power).

The best places to look for more performance in a 912 are small upgrades: moving to lightweight wheels if not already fitted, better tires, lightening the car where you can, and improving your own driving skills... all of these are less expensive than looking at the engine, and will have much greater impact on the car.

As for it's value... an all original longhood 912 in excellent condition can command some decent money as they've become increasingly sought out by collectors. However you are dealing with a car that has been repainted and one mini cn900 key programmer that has been converted to the later style bumpers. While these might be good upgrades in practical terms, they will make the car a non-consideration for any collector. If you have documentation on the repaint from the shop (ideally with pictures of it being done to show how it was done and that any rust issues or such were addressed properly and not just covered up), the devaluation from the repaint will be minimal. But lacking that and the bumper change, it could be in the $4-5k range if cosmetically good and in good running order... maybe a little more for the right buyer, but also could be less given current market conditions.