Luke, drop Archie an email, asking for his prices on the YS engines. I'm sure he is itching to tell you but cant advertise on here! Ask if he can do the 140 FZ sport. I think you may be surprised at the price if he can. And these come with a silencer included, though you may struggle to hit the EMC noise limits with just that.

As for cheaper silencers than Hatori for the YS, again Just Engines and Probuild as mentioned do alternatives.

I have seen an older YS 120 (SC at a guess) go fine on 10% formula irvine, it just takes more tuning as Archie says.

Oh decisions decisions.

Of course if you are really stuck, I'll sell you my 120fz (including in cowl muffler) and buy a brand new 140