You can get motor mounts to bolt a 1000 triple in any of the ZR, Mt. Cat, 1M bulkheads. A couple places also build pipes for that swap. Those would also work for any of the 800 or 900 triples. A 600 triple will fit in anything but the '01+ Mountain sleds, and with some work, I'm sure you could get one in there.
The 1050 that Ridecats was talking about is based on a polaris watercraft crankcase.
The 1500 that I ride is two 900 twin motors cut and welded to make a triple.
From '99 on, the triple bulkheads were the same as the twins (until the firecat/M/F chassis came out) The mountain sleds were a little different.
My 1500 is a 1M chassis, but it has a T-cat bellypan and hood on it.