I personally have never heard of that plastic piece deteriorating enough to leak before. Doesn't mean it doesn't, just nobody reported that condition before now on here and I'm afraid that the only remedy is a new sending unit - Epoxies and J-B Weld might get eaten up by the gasoline over time and you would be right back where you started.
To the point of buying one, once you get the existing one out of the tank you can put an ohmmeter on it and swing it stop-to-stop to see that the resistance is at empty and full to really know what it is, then go to the VDO site to look it up and get the VDO part number. I saw that JEGS is listed as a dealer on the VDO site that Luis provided up above, so that might be a good place to start, along with other dealer sites from the VDO website to get a sender with the right resistance range for your gauge head and that fits a 1970-ish VW gas tank.
Or you could call Greg Leach at VS for his input and maybe get a sender from him.
Sorry, That's all I've got....