The circuit diagram for the amplifier can be found here:-
https://www.audiomisc.co.uk/Armstrong/600/600faq.html
I think the transistors that have failed are the ones shown as items 623 and 624 - 2N5320 and 2N5322. You may have better luck trying to find those transistors rather than the RCA ones. The 2N ones are in a round can - unlike the flat package your existing transistors have.