I would doubt if Oracle published its internals for 3'd party vendors to build GOOD replication engine. (If however it is so, I would like to know this API...)
A while ago I looked through docs about replication engine from Sybase but the documentation was not clear to say least. Nor it was clear how they intefaced Oracle for this task.
The issue is that to build replication one needs to serialise user transactions, i.e. to use Oracle's serialise transaction mode (this is quite expensive in terms of storage), or need access to some Oracle internals (transaction SCN).