I was able to find a way to replicate much of what the Subscriptions plugin does using Invoice Ninja, which supports a multitude of payment processors including Paypal. I highly recommend it!
The Invoice Ninja “payment links” documentation page is a good place to start if you want to try this: Payment Links | Invoice Ninja
Then you use a webhook and scheduled cron task to make sure discourse is updated when people start, change or end their subscription plan.