I'm sure I've got some spare TR3 heater motors around, but I'll have to hunt for them. No reason you couldn't have one rewound, it's just a rather pedestrian little series-wound DC motor ... but it's a rather labor intensive process and I expect the cost would be prohibitive. I've rewound the armature myself; first one worked great but I never managed to finish the second one ... it's been laying on the back of my bench for over 10 years now /bcforum/images/%%GRAEMLIN_URL%%/grin.gif Hardest part was finding the proper insulation to put between the windings and the armature.
No O-ring in the stock heater valve, it was just a bare brass cone against a bare brass seat. Not intended to seal without a drop, but shouldn't flow enough to notice through a heater. But there is supposed to be packing for the valve stem, which you can replace with "valve stem packing" from any good plumbing supply (even HD had it last time I looked but that was some years ago). I'd suggest the good stuff with the PTFE coating, as it's more forgiving of worn stems IMO.
Others have said it works OK for them, but I've never had much success using ordinary heater hose on a TR3. They always kink where they turn 90 degrees to go onto the heater core, and block most of the flow. Hopefully the ribbed ones from TRF will hold up better ... I solved the problem by adding copper elbows to make the turn.