Parting things out nearly always nets more money, but whether it’s worth your time is the question. I think splitting the engines and chassis is a good idea. The engines can still be run in different part of the country, they are still popular in the midwest and if nothing else, someone might want one for a practice motor.
The chassis will be a little harder to sell because it has older “breadbox” bodywork, an older rear bumper setup and so on. Most tracks won’t let that race with other karts, but many will let you turn laps for fun and practice. Converting them to the more modern design can be more hassle than it’s worth. Your best leads for selling the chassis are folks that are looking to run 90’s style karts, like myself and @Mynameismcgyver, or people looking for an inexpensive chassis to run a briggs 206 on and either want to spend a little time converting it to modern bodywork or race at a track that has more liberal bodywork rules.
Figure 400ish for a motor, 300 for a chassis. Like anything, depends on the market etc.
Here’s some resources to trade karts: