Welcome to the forum and mud mowing!
For mods, I'd swap out the front wheel bushings. They get sloppy after awhile no matter how often you grease them and the wear into the spindles, sometimes leaving a 1/4" deep gouge. I like to swap in ball bearings and change spindles if needed, the bearing size is 3/4 x 1 3/8. You can find a set on amazon for around 20 bucks so I consider it a good upgrade seeing as your spindles will no longer be wear parts per se, and the wheels no longer need grease if you use sealed bearings.
Tire upgrades are welcome, but not necessary. Ag tires are great in mud and loose dirt, knobby tires are best for semi-packed dirt, and deep-lug tires like Kenda Executioners and ITP Mudlites are best in deep, soupy mud that would sink your boots.
Also, it's good to look at your steering play and if needed replace the steering gears, they also wear out.
Some people like to lock the transaxle, but if you're getting started I'd hold off on that for awhile because it's best to have a good working knowledge of transaxles before splitting the case. Here is a locker design I recommend: https://www.atltf.com/t2789-locking-a-peerless-transaxle-the-doc-locker-video-added-2-23-14?highlight=doc+locker#34554
Doc also has a weldless variant as well if you don't have a welder.
If you have a craftsman this isn't necessary, but if you have a welder, I'd reinforce the frame if you have a lot of uneven ground, seeing as they can flex sometimes in the frame if the front axle bottoms out.
Conversely, if you want to go over 20-25 mph regularly, I'd recommend locking the front axle pivot for stability.
Also, front bumpers and grill guards are great because stuff happens. LED work lights are nice if you plan to be out after dark or you just want something that looks cool. And finally, a winch is nice
Hope this isn't too much to process, good luck with the project!