The difference between the GFS and the IFS is that the GFS is the glow version and the IFS is the Gas version. The GFS-W is a glow engine with a Walboro Carb although all three in the pictures you are looking at are the gas versions. Just Engines out of the UK has these for quite a bit less than Morris, the gas version(IFS) for just under $300(once the conversions are done) and the glow version for about $180, they also have a good reputation for service and are always willing to answer questions.

http://www.justengines.unseen.org/

This site is also a good source for informatioin as well. He is very helpful and knows a lot about the MVVS line.

http://home.wanadoo.nl/pereivers/home.html