DT125R FORUM

The missing battery is likely a bigger deal than you think on this bike. Without it, the regulator has nothing to buffer, so voltage can spike and drop as revs change, and the CDI won't get a stable supply. That alone can cause a weak, erratic spark even if the coil and stator test fine. Get a known-good battery in there first, then clean and check the main earth points at the frame and engine, not just the coil bracket. The ziptie wiring from the previous owner needs tracing properly. Fix the basics before chasing the carb again.

The window-and-break-it-apart approach is worth considering, but be careful with the debris. If any fragments pull back into the cylinder you've got the exact failure you're worried about, so I'd stuff a rag or vacuum the cavity thoroughly before closing it up. On the originality concern, a patch welded underneath is easy to hide and keeps the stock look, which fits your plan to sell. Whether the cat alone is the restriction or there are internal cones too is something I can't confirm from here, so it'd be worth checking before cutting.