quickest way to prove anything on a chassis is dummy load the horizontal output of the flyback, best way to do it is remove the HOT and add a 40w lamp to the centre pin track and ground, if the lamp light up then you can read the b+, if thats all good and the HOT test ok then likely the issue is the drive signal for the HOT which normally going to be an open circuit coil or a faulty ic. If the lamp does not light then you desolder the b+ pin of the flyback and add the lamp to the track of that and then if the lamp lights up then flyback is probably bad
Korteks suffer from bad flybacks and also due to age I expect a fair few caps will need replacing, also worth noting are hv caps in the horizontal deflection circuit which can cause a mutlitude of issues - from killing the HOT to width issues to ocp issues to excessive HV
if you are not technical then don't attempt any of the above due to dangerous voltages, just send to someone for repair- you have quite a few repair companies in North America