Just some thoughts from an experienced member to a new one...
If you can buy a brand new light for $25, it's not worth it to fix an old one, especially if you value your time. If you are looking to improve it, that could be a different story.
If you chew through a lot of batteries, it could be worth it to install a 32650 or 26650 LiIon cell instead of alkalines. You'd get increased run time and recharge ability. By the time you throw out 10 alkalines (two at a time), you could have bought a LiIon. After that the LiIon is free for a long time. If your alkalines last 6 months to a year or more, you should probably ignore the rest of this post.
These cells are longer than D cells, so two don't usually fit in the stock light body. Fortunately, one is usually a little better than two alkalines. If your light has an enormous spring like a MAG, you may be able to modify it to accept two of these cells, which would enable huge performance improvements.
There are xx600 cells that are the same length as D cells, but they often represent significant compromises over their xx650 bretheren. Note that the '60' and '65' represent the length in mm, while the final '0' indicates a cylindrical cell. It could be worth it to look at cells like this if you want extreme performance in a body that can't handle the longer cells.
If you want to improve the light output, the increase in runtime gets much more significant. Alkalines suck in high-output applications, where LiIons excel. "High output' for an alkaline is anything that will drain the battery in less than a few hours of use (this is very subjective). Looking at the Coppertop datasheet, for 2W out of the 2 batteries to get 300-ish lumens (optimistic to the extreme) the D cells will give you about 6.6W-H each, giving 6.6 hours of runtime, assuming 100% driver efficiency. To run just 4W, the D cell output drops to 1.5 W-H each , giving you only 45 minutes of runtime! You would have expected a bit less than 3.3 hours, but the increased load destroyed the alkaline's capabilities. Most LiIons would barely notice this difference.
Alkalines have their place. There are a number of them in my house. They are NOT in flashlights that I use very often.