You can buy a barrel and upper from Tactical Solutions that will fit your lower grip frame.
http://www.tacticalsol.com/tshome/ta...series-pistols
As someone said, the RST is a Ruger Standard, the Mk I was a target model in either 5 1/4" heavy tapered target barrel, 5 1/2" bull barrel or 6 7/8" heavy tapered target barrel. the MK Is had adjustable sights where the standards had fixed sights.