When you say , "something more modern" are you talkin transistor electronics to replace the tube amp ? Are you planning on using the record player at all ? Cause if you are , the mechanical portion (the turntable) is gonna need a lot more work (read $$) than the amp will . The amp only should need it's capacitors replaced , but the turntable will need clean/lube/adjust , new pick up arm cartridge (which is just about assured to be dead , these die just like capacitors do , from age and sitting) , and it's rubber drive wheel (or belt) replaced . Also , if the amp's tubes only light on 78 RPM and not on 45 , that's a fault of the turntable itself and not the amp !
Clearly stating your final objective for this piece will greatly aid in giving appropriate answers