Aspirin is for us analgetic but for birds very toxic, as is most medications for humans. Aspirin should never be given to birds in any case. Viral diseases can not be treated, there is no cure for a virus, except vaccines. So no matter what you give the pigeon it will not cure the pox nor will it speed up the recovery. Antibiotics kill bacteria, not viruses, but they also suppress immune system, so they should be given only if necessary - if the pox nodules get infected, because antibiotics will slow down the recovery from the virus.
In case you really don't have any way to get a bird specific antibiotic, and you really need it, you can give Amoxicillin with clavulanic acid, 30 mg a day for adult, 15 mg for a baby.But this only if there are secondary infections.
As for the virus it will run its course in two weeks, so best thing you can do is maintain hygiene, make sure the pigeon is eating and drinking, keep it where it is warm and where it can rest and be not bothered by other pigeons, animals or humans.