I know Miyata 1000's were available in 50 cm. Probably not the best choice, though, because it had a long top tube. If I remember correctly, the top tube on my 50 cm 1984 Miyata 1000 was 53.5 cm, or in the same ballpark as my 21"/54 cm road bikes.
A small (~15") rigid mountain bike with 26" wheels might make a good starting point for drop bar conversion. Some hybrids are a good match, too. There are lots and lots of old Trek Multitrack 720's out there whose geometry is a pretty close match to Trek's touring frames.