The old tether planes died because RC is so cheap now. The Cox .049 engine is gone as well because brushless electric motors are cheap and much easier to deal with. If you want gas-powered fun, there are plenty of engines available for cars, boats, and planes, but they are bigger than .049. They still use the old glow plug ignition system, but the new carburetors work much better.
Don’t mourn the end of the old tethered plane with an .049 engine. Rejoice that technology has given us much better toys!!
I agree ... with electric power at the low end, gasoline power in the middle, and real live turbo-fan engines at the high end, the "good ol' days" of remote controlled toys are Right Now.
As with all small displacement glow engines, the trick to reliable operation is to keep from trying to leaning the needle valve to get every last rpm out of the engine.
When I was younger, my brother and I would get flight after flight out of our .049 engines simply because we knew how to properly tune them. The engines started on the first flip and ran until they ran out of fuel.
Even with larger carburetors, it's possible to over-lean the mixture and cause unreliable operation. The real secret is to set the needle once, then leave it alone.
I meant to add that control-line flying is still alive and well and as popular as ever, even with the advent of R/C. So is free-flight.
The only real difference is that there are a lot more people participating in the hobby because the advancing technology has allowed everyone to pursue whatever aspect of model aviation that they find interesting.