Tom,

get an opinion from a user of bothe engines before deciding.
I do not have any experience on MVVS, but I have to say that OS 91 FX in the second sweetest engine (after OS 1.40 RX) I have ever used.
That includes tens of engines in 25 years, eg. OS, Irvine, Webra, TT, Enya, Saito, HP, Super Tigre, YS 2c, YS 4C etc. etc.

My 91 FX has given me only two dead sticks: once the plug got blown (you should replace it at least once a season) and the other time was when I tested Cline regulator (it did not work).

With the stock muffler you might need a pump but with a pipe it is fine (more tank pressure). With a small restricting muffler (Pitts etc.) it runs hot and causes problems.

ini