I've got the motobecane sscx bike, I've rebuilt the wheels with lighter carbon rims and butted spokes which has gotten it down to a chunky 26lbs, it isn't light by any means although it rides wonderfully and was affordable. I suspect you're not seeing track frames because they really won't come close to handling anything resembling a cross tire. Looking at my Felt TK2, a common track frame around here, any my daughter's Javelin, even shrinking down to 650b the space between the stays would be tight. They both start with only 120mm rear spacing and then slightly curve the stays inwards. Both have very tight clearance to the frame, and both struggle to run a 25c tire on a 19mm internal width rim, they really are designed for 23c tires. The motobecane has the "nice feature" of 120mm rear spacing so it'll easily accommodate a fixie wheel and run tracklocross, it also means that the selection of 120mm disc hubs sucks big time. There are other single speed cross frames but they're typically 135mm spacing so no fixie hub will fit well. Going with an 80s, horizontal dropout frameset is probably the easiest was to go.