Early Magnatones used basic radio amplifier circuits and common radio tubes, like the 6V6 and 6SJ7. Like a lot of post-war amplifier companies, Magna used war surplus tubes, and would buy large lots of tubes. If a circuit used a 6N7, and the re-stocked 6SN7 or 6SC7's, they tweak the circuit for those tubes. I've even seen Magna's with 7C5 Loctal tubes (a 6V6 with a different base).
When the 200 series came around, louder amps were required, with slightly more sophisticated circuits. The 6CZ5 and 6973 were used. In 1963, engineers redesigned circuits to use the GE 7189A and 6GW8 tubes. To maximize buying power, nearly all Magnatones were designed ot use one or the other.
In general, guitar tube amp guru's would say that any 50+ year old amps with original tubes should be treated to a tune up and be given new tubes. With Magnatone amps, I don't think that blanket statement applies. If the amp sounds good it is certainly unlikely that it needs pre-amp tubes. Preamp tubes configured for gain general draw a low current and don't wear out. The sixties Magnatones were built with high quality Mullard preamp tubes. I don't think there is a new production 12AX7 that can match the audio fidelity of those original tubes.
Power tubes can be a different story. They do wear out. With Magnatones, however they don't wear out like they do in Fenders. Magnatone engineers tended to stick to the maximum limits as given by the tube vendor, whereas Leo would drive 6V6's and 5881's well past what the tubes were rated for. If the amps sounds good, the power tubes might very well not need replacing. Part of my hestitation is that these old tubes are no longer in production, excpet for the 6V6, 6L6, and EL34 tubes. Current production examples of these tubes are of good quality and are generally well thought of.
The same goes for rectifier tubes. 5U4 and 5Y3 tubes are in current production, but other rectifier tubes, like the 6X5GT or the 5V4. My recommendation on replacing 5Y3 tubes is to use an NOS 5Y3, and not a new production 5Y3. The new 5Y3's meet and actually exceed the original specifications for the tube, and deliver a bit more B+ voltage and don't sag like the originals. NOS 5Y3's are not that much more expensive that new ones, so why not?
Phase Inverter (PI) and Reverb driver tend to be used in circuits that draw more current and are more likely to wear out. Early PI circuits were para-phase designs, and the PI tubes were actually both gain stages and did not draw a lot of current. Later (1955 and on) were cathodyne circuits, which do draw more current and will wear a tube out. Luckily enough, most of those amps used 12AX7s and 12AU7s for that duty, and those tubes are readily available. The tube driven reverb tanks for the early sixties designs put a heavy draw on those tubes, so replacing them is likely a good idea in an amplifier health check.
One caveat to all this is if you are a touring musician. If you are touring with a fifty year old amp, you've got your own tubes tricks up your sleeve, or a witch doctor tech that tours with you, so probably aren't even reading this!