Yes, this pitless just sort of wedges itself inside the casing, and relies on a metal bar that sits across the top of the casing to suspend it. I called the manufacturer who was unable to give me any details about the seal, but referred me to a local distributor who couldn't find anything about it. So looks like I'll be using the existing one. It's in good enough condition though.
I've got my PVC push-pipes 163.5 out of 165 feet down. Cut though 3 pieces of tape, and have one left it seems. This one has been a bit stubborn, but I expect I'll get it soon enough.
One strange thing though - There is a broken wire end down there. The pump still runs just fine (2 wire, 230V), so it's not one of the supply lines. It is hard to tell, but it seems to be a smaller gauge stranded wire, probably with a yellow (or other lighter color) stripe on the insulation. I'd guess a ground wire lead from the pump. As far as I can tell, there were only ever 2 wires going down the casing, so maybe it was just left disconnected when installing.
I assume mine is something like this:
https://www.ebay.com/itm/204381685173
It has strong looking attachment point for a safety cable. If I could get a hook around that I could pull a lot harder. Would be pretty difficult to get something on there at that depth though, and next to impossible to do so if the drop pipe or wires break.