The 77 Monte Carlo Easy Kit has some Halibrand that are 19-21". If you remove the mounting bosses they fit perfectly into the Pegasus 23" step sleves.
The other option is that I can "scale-Up" some 1/25 Mini lites by making a mold from the Micro Mark Blue rubber and then letting it set in Coleman fuel for 2 days. This will give you 1 master that will be larger.