With no vet available your options are very limited.
I would put the snake in a very sterile environment - just an enclosure lined with clean paper and a water bowl. Fortunately this boa is large enough it doesn't really need a hide.
Silvadene (silver sulfadiazine) is the go-to antiseptic cream for reptile injuries. Even if your local vet doesn't see snakes you might be able to purchase some from him - or it may even be available over-the-counter outside the US.
If you can't get it there's a product called Vetericyn that is also good on reptiles, you can find it on Amazon.
I would also feed very small feeders so as not to stretch the area while it heals.
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