3. By shaft seal do you mean countershaft seal? If that's it, just remove the sprocket, put a screw (wood) into the seal and pull it out. Maybe two screws 180 degrees apart and wiggle it out. Put some tape over the countershaft to protect the new seal and push the new seal in by hand or use a wood dowel and .
2. There's not much you can do with the front wheel without some custom hub work or a custom wheel. The simplest thing is to find a tire you want in the 8" rim size. For the rear, replacing the stock hubs with something that has a 4-110 pattern will open up a whole new world of choices in wheels and spacers. I wouldn't go too wide on spacers if you ride rough trails or get any air. The stock axles aren't tough enough to prevent bending with the wheels way on out there for hard riding, unless you're a sub 150lb rider. Straight axles with good hub splines aren't easy to come by anymore.
If you want wheelies, make certain the rear brake is in top notch condition, which includes a good cable and brake lever pivot (clean and grease the pivot).
1. There aren't many domestically available aftermarket mufflers that aren't loud to the point of being obnoxious, especially when on a group ride to the rider behind.
Here are a couple mufflers that may be what you're looking for.
https://www.ebay.com/itm/11565597404...temCondition=3
https://www.ebay.com/itm/26288275391...temCondition=3
The only reproduction option, for a full exhaust, I know of is Musket Mufflers in NZ. They are expensive.
https://www.musketmufflers.com/
The story of three wheels and a man...