Welcome!
First, are you performing service yourself? If so, I suppose you are aware of high voltage and that you are capable of working on such apparatuses...
You can use electrolytic caps of tha same value and same or higher voltage as the schematics say. Some like to use large MPL caps, and you can, but my tip is to stick with whatever that is closest to the original design.
As for the rectifier/higher voltage, if you have too high b+ with semiconductor diodes, you either have to arrange the filter caps so that they can handle the voltage or switch back to the tube rectifier. However, if your amp does not have a 240 volts input option, you are in troble anyway. It is a common problem, where I live we have 240V and some have 260. Check your heater supply, you need to have around 6,3VAC for the tubes to function properly/wear naturally.
There are numbers of solutions. I am sure the experts here will chime in, who have more time on their backs than me. What I like to do, is to use a bucking transformer. Some use resistors etc (or diodes, as yo say?) but I do not recommend it. Of course, you can use a 240 to 220 volts transformer, but it would need to be quite large. If you use a bucking transformer at 240-20 volts, you can get away with a much smaller transformer.