I’d say the best answer is “it depends”…
If you’re running the 2-cycle stuff at the SKUSA Pro Tour / FWT / Gatorz level, the newer, the better. There’s a reason many of these guys have a new (or near new) chassis every weekend – I don’t think it just has to do with deep pockets.
If you raced those classes at the Regional level, it’s a little less important. At the club level, it becomes much less important. The top Rotax racer at my club won a ticket to the Rotax Worlds last year on a rented kart for that weekend. At the club level, he runs a 5 year-old kart, and wins with his talent – his club kart would be a bit of a liability at the higher level though.
That said, if memory serves, a few years ago Jordan Musser showed up at the US Rotax Nationals with an older kart, and won. So bigger events, even in the higher-hp classes, can be won by older karts.
If you take the LO206, and look at what the class is really focused on, it’s focused primarily on the club level. In this case, you can absolutely take an older chassis and win with it. At the club level, it’s mostly down to driver skill.
Where things might get a bit tricky is comparing WKA rules to other (IKF or ASN Canada) 4-cycle rules, and what impact a newer chassis, or a chassis designed for a 4-cycle, might have on things at the higher levels.
It’s been proven at the WKA Gold Cups that an older kart can win at the top levels. In Canada, at the Nationals, I don’t know if you’d see this happen or not – while the karts might not be brand new out of the box, they’re normally pretty new, and designed for a 4-cycle.
In any event, I think it would be safe to say that a 4-cycle wouldn’t put the same stresses on a chassis as a TAG or shifter would, in terms of fatiguing the frame.
The biggest problem with older karts is that if you don’t know the history of the kart, you can often spend a lot of money straightening the frame, replacing bent components, brakes that leak, etc. A lot of the time, it can cost less in the long run to buy a newer kart over an older kart.