As in heads of martial schools, or masters of martial schools. I suppose "founder" can do, but it doesn't sound right. "Elders" is taken by another word, and "master" only seems to work for a specific master, so I don't think that's right either.
If you're going to go with a romanization of the English such as "sifu" you should probably go with standard Pinyin (i.e. Shizu）, rather than some other lesser-used romanization system like Wade-Giles or Yale. Perhaps "Grand Masters" of "Founding Masters" might work well in English. The term seems to be plural (although I have no context) so make sure you use the plural form.
In English, I don't think they really worship the founder as we do.
I would recommend two ways to show the seniority and hierarchy of 师祖.
1. Mask his name. Like Voldemort, everyone calls him "the one who cannot be named". You can figure out A shorter version
2.use his name to name some important things. Say Newton's law
It feels like you are talking more about 掌门 or 掌门人 when you are referring to the master of a school of martial arts which is something more like "Head" (as in headmaster), leader or even chief。 Of course this is more of a translation in the context of a martial art sects/schools but can also be informally interpreted as someone who is in charge of an organization (such as CEO or government leader).
堂(表)兄(弟/姊/妹)all translates to