[Arran]

The other electrolytic can, the one under the chassis, can be replaced by three poly film caps if desired as they are small values. Whether they would fit inside the original can is another story but they are available and will survive much longer then replacement electrolytic caps. They can be purchased from Radio Daze and Mauser.
With regards to a positive side going to ground this is something to be careful with, there are some filter caps that have a common positive with two negative sides where a bias resistor network and speaker field coil will be connected in series with the negative side, the field coil acts as a filter choke and the resistors as a bias voltage divider.
Regards
Arran