Before giving up locally, consider scouting around the TRACTOR fraternity. Continental engines were used in forklifts, mobile generators and irrigation, but especially Massey/Massey-Ferguson tractors. >>There is nothing rocket science about the engines themselves<<, and given a workshop manual, anyone who has rebuilt a flathead 6 iI, say, a Plymouth of Dodge can rebuild these. Offhand, about the only different bits in a 54 Manhattan per se compared to the other Continental engines in Kaisers and anything else are the valve rotators and the supercharger/carburation set-up. Buying a workshop manual--or just even getting copies of a few pages of the necessary specs for valve adjustment etc is really all that is needed--or should be needed.
True story: My 49 Kaiser had a completely worn out engine when I got the car--it would run in warm weather and didn't smoke, but had been used up otherwise. I live in a hamlet of 50 people, the nearest village in a rural farming area is 7 miles/11 km away, and virtually every single part (over 95%) was on the shelf of a car/truck/tractor parts place in this village--or next day delivery from a regional warehouse 90 minutes away for the few other bits. The next day, every single part was there. It helps that Massey and Massey-Ferguson were popular tractors around here. Local fellow here has rebuilt a LOT of 4 & 6 cylinder tractors with Continentals, and mine he could do blindfolded. The rebuild--new valves, pistons, boiled out block, seals and gaskets etc was not expensive. It helps if you use the names Clark and Massey and Continental as well as Kaiser, when you are asking for expertise.
It is exactly like rebuilding King Pins--if a modern garage doesn't know them, take it to a big truck or tractor place!
Hmmmm....are you talking to people who can only find parts if the have the right order number for the computer, or are they 'scared off' by the weird & unknown to them brand name of car and automaticially saying 'no'? Just saying...