The most important thing is having a good running machine with freshly gone through suspension. Lots of the old trikes have worn out suspension. Getting it back to at least stock performance is a must, or your frame will crack up.
If your machine is running well, all you really need is a kill switch. If you want something a little more race ready this list will get you started, and if you are a good rider, you could be pretty competitive.
1. Kill Switch
2. Get comfortable handlebars
3. 18 inch tires. Kenda Scorpions and ungrooved Turf Tamers provide some traction and slide easily.
4. Extended Axle, Lone Star, or DuraBlue an help you.
5. Performance exhaust, remove airbox lid, change jetting.
The next steps would be custom suspension, followed by more motor mods if necessary. You don't need inverted forks or big bore aftermarket cylinders if you want to do vintage classes. Those mods are generally limited to open classes. In three wheeler motocross, a little talent and hard work will go a lot farther then a ton of aftermarket parts.
Most of us who will be racing Haspin, have raced together in the past. We tend to be pretty respectful of one another's space on the track as locking wheels is about the worst thing that can happen. I would be more worried about getting hurt in a quad class due to their level of enthusiasm vs sense of self preservation. We try to keep it as safe as possible, all things considered.
Would you rather race the big or small track? Check out the videos above if you aren't sure.