Not so big a deal, as awful as this is (yikes, a 555 in a Tektronix? An open loop switcher at that?), it's not huge, and an adequately sized tant will do.
The caps after the resistor don't see much ripple so it doesn't matter much; C4 is the critical one, seeing full switching ripple, and also being a smaller value it could stand to be a lower ESR type. I don't recall that there's much selection in dipped leaded tantalum, so you might have to go with another type if you do need to replace that one.
Agreed, 50V is better than 35V rating.
The modern substitute would be ceramic or aluminum polymer. And the ESR doesn't seem to matter here, as there's no compensation loop to be weary of...
Electrolytics are even available in good ESRs at that value, but I wouldn't recommend them overall.
Tim