LED Headlight - If you are using your BIG RED at night or in a low light you may want to upgrade from the original incandescent bulb to an LED light. The improvement is substantial and the 4 X 6 LED lights are readily available and affordable. Below are some links to forum posts that provide details of the upgrade.
Stock headlight high beam
LED headlight high beam
Sharpen Footpegs
Many BIG REDS were used for work and after all that is what they were designed to do. Many of the BIG REDS that I have owned had dull worn down footpegs. It just takes a file and a few minutes to sharpen up the footpegs. Sharp footpegs just feel good since you will be more sure footed especially if you have muddy boots. Sharpen just enough for good grip. You don't want to sharpen so much that you will tear up your shins or cut up your boot soles.
AGM Battery
Battery technology has come a long way since the BIG REDS were new in the 1980's. The AGM (advanced glass mat) batteries are not the current cutting edge technology but they are typically more powerful and last longer than the standard lead acid batteries. Having a good AGM battery in a BIG RED has been great for me. My last AGM battery lasted 6 years and provided consistent starts even in the coldest temperatures. Note that there's many different AGM batteries out there. When it comes to these batteries I believe the old saying of "you get what you pay for" applies. Do your research before buying an AGM Battery. I have had good consistent luck with Yuasa batteries.
Grips
Adding new grips is along the same lines as sharpening your footpegs. New grips just feel good and provide a more positive confident grip as compared to worn grips. For me it makes the riding experience better. New grips are affordable and readily available. At the time this post was written the original OEM Honda grips were still available.
Grip Heaters / Handwarmers
If you live in a cold climate adding handwarmers is a great upgrade. Warm hands after a long day cutting firewood or hunting will get you thinking why didn't I do this a long time ago. The installation is fairly simple and below is a link to a forum post that gives you the details.
Thumb Warmer
Same benefits as handwarmers listed above but this is for your right throttle thumb. You may think that handwarmers are enough but trust me add the thumbwarmer when you add the handwarmers. There's a reason why most snow machines/snowmobiles have both hand and thumb warmers. Details of this upgrade are included in the link below.
Fork Oil
This applies to the 1985 -1987 250ES BIG RED models. Changing the fork oil could be considered as a maintenance item but I am listing this as an upgrade because most BIG RED owners don't do this. There hasn't been one Big Red that I have bought that didn't have terrible looking fork oil when I drained it. The fork oil is not the easiest to change on the 250ES Big Reds as you need to remove the forks from the machine. The forks require a certain amount of oil to operate correctly (give the most comfortable ride as the design allows). The fork oil breaks down with use, contaminates get in, and oil can leak out over time. This can lead to excessive wear of the forks internal parts so changing the fork oil can provide a more comfortable ride and keep the fork internals in good shape. The fork design has a small amount of travel and a small quantity of oil so maintaining the proper amount of good clean oil is important. As I mentioned earlier my experience is that most of the Big Reds I have worked on didn't have proper fork oil maintenance so when the forks were disassembled, cleaned, and reassembled with new fork oil the ride quality improved quite a bit. If you plan on changing your fork oil I would look into buying new fork seals and new fork boots as they may be necessary. Below is a link to a forum post and a video on changing the fork oil.
Brakes
Brakes are in the same category as the fork oil change. It could be considered a maintenance item but since most Big Reds I have worked on don't have functional front and rear brakes I am considering it an upgrade here. Let's talk about part quality. Many of the Big Reds I have worked on had the cheap amazon/ebay brake shoes on them. Upon disassembly I typically find that the bonded lining has fallen off the shoe. In some cases the dislodged lining jams up in the brake drum so the wheel can't turn. I have used EBC and Vesrah brake shoes and they offer good braking performance and the linings stay on the shoes. Having a properly functioning front and rear brake on your Big Red is necessary in my opinion is a necessity. I have multiple Big Reds and in many cases friends and family ride these machines with me so having good brakes gives me piece of mind. If you are going to be changing your brake shoes take a good look at the condition of the brake cables, levers, and adjusters as they may need replacement too. Below are some forum links that provide details on brake shoe replacement.
That's eight upgrades that provide a good "bang for the buck" in my opinion. I hope this can help some of you out there enjoy your Big Reds more.