If the Hi-Lo your looking at is in fact, that old, the factory probably wouldn't have any repair parts on hand anyway. After about the first 7 or 8 years of ownership, I was my own for locating repair parts for our 1969 Hi-Lo Bon Voyage. The good news is, as kwdstalker says, most all the parts in a Hi-Lo aren't made by Hi-Lo, and can be purchased "off the shelf" at RV service and parts centers.
Having written this, I have to admit that during the 40 years we've had our Hi-Lo, I've never had a lift system problem. Although this is a good endorsement for the system's reliability, correct operation, and regular preventive maintenance, it also means that I've never had to locate a lift cable, hydraulic pump, ram seals, etc... (The stuff not carried by an RV service and parts center.)
I'd be very interested in finding alternative sources for parts like these that truly are unique to Hi-Lo trailers, just in case.
1967 17' Hi-Lo Bon Voyage (1967-1969)
1969 19' Hi-Lo Bon Voyage (1969-2011)
2004 27' Hi-Lo TowLite (2010-present)
2015 Chevy Tahoe LTZ