Vintage Monitor top Refrigerator

Hi All.
I am no engineer by any means. But here is what I do. I restore old monitor top refrigerators.Attachment 14339 The ones I am working on now have or had Methyl Formate in them, which they used as the refrigerant. 1934-39. I am having a lot of trouble getting it. I been investigating and reading that R601(pentane) Might work? it is close to the same properties MF has.
Once the system is open I installed a .028 x 40" capillary tube to replace the float valve on the unit .When you do this you can install the Methyl Formate and the unit will work fine. These old type compressor only run around 1750rpm and there built like tanks. They used MF for a few years then switched over to R-12 around 1939
Also the compressor oil is a lard base oil they used back then. The other forum I belong to Monitor Top Refrigerator Forum http://monitortop.freeforums.net/ has lots of information that helps a lot. But I just need more information so I came here.
I guess what I am asking is does someone maybe have a recommendation on another refrigerant that is easy to get here in the US and be able to use that would be compatible with this old type of refrigeration unit such as the Monitor top and a compressor that only runs at 1750 rpm. Any help or information will be greatly appreciated.
Thanks