That's exactly what runs at https://ripple.com/client/ yes.

The server component it connects to (see https://ripple.com/client/#/options) is the server called "rippled" from https://github.com/ripple/rippled. If you want to modify it so it uses BTC instead of XRP as native asset (I'd recommend adding BTC as alternative instead of replacing XRP, but that's up to you) you need to work on that first, not the client which is more or less a dumb frontend for an untrusted API provider.

Just like with Bitcoin alt-coins just please think of an original name for your project and use that throughout your fork, similar to every coin that is not on the Satoshi block chain should not call itself "Bitcoin", any distributed marketplace that does not run on the OpenCoin/RippleLabs ledger chain should not call itself "Ripple".

It would be really cool to see some forks in the wild by the way, if you need any help I'd recommend jumping over to the ripple.com forums though, the churn rate in this forum section (+ the amount of "DOGECOIN IS TEH AWSUM! WOW ! SO COIN!" posts) is a littel bit too high for more technical discussions.