the lower removable rail is not available as a separate part from Suzuki, you have to buy a full frame !!!!! the mk1 1200 has bolt on engine mount the 600 has a welded engine mount, im sure they will fit but wouldn't look right they do come up on the bay or you can weld in a new section.
Suzuki GSF1200 Bandit MRK1 Frame Rail Section | Eblag