Polypropylene has the advantage of being self healing which means that if the voltage limit is exceeded and there's an arc-over, the capacitor will not be destroyed completely, the capacitance will just drop a bit and the leakage current might increase a bit.
I don't know whether polypropylene is a requirement because I haven't seen the schematic but it's a good idea to replace like with like. Of course you can use a higher voltage rating, the higher the better.