Hi and welcome
Without knowing what the lump in question is exactly, how large and where exactly it is located, we cannot comment on what is best done, Any lump needs to be seen in a hands-on examination as there are is quite a variety of them around. This is one of the limitations of an online place like our forum.
I would recommend you to see a vet who is more experienced with guinea pigs and more confident on operating/dealing with lumps of all kinds. A vet appointment with a specialist vet outside your area does count as essential travel during lockdown; it in the spring and does now. You are always within your rights to get a seond opinion; especially in an are where the first vet is not very confident. Any drained cysts will refill again, so this is not a long term solution.
Here is our recommended vets locator to find one within your reach:
Vet Locator
Accessing veterinary care during coronavirus/Covid-19 lockdown
PS: An operation with a vet experienced with guinea pigs and/or small animals (exotics, as which guinea pigs are classed as) has a much higher chance of running without problems. Overall, we have seen a massive improvement in the operation success rate as they have become much more common and there is therefore more experience, practice and confidence around. But I would strongly recommend not to use a vet who is not confident and happy to perform an operation on a guinea pig unless there is absolutely no alternative.
Tips For Post-operative Care