I've got a hankering for a Tunnel Vision. The main thing, other than my checkbook, that is keeping me from getting one is not knowing what engine to put on it. The manufacturer likes the OS 160. I have no experience with the OS 160, but from reading comments from other's I am not excited about using it.

I would like to put a four stroke on it, but it would be nice to get some advice from some of you that are more knowledgeable about these big profiles. Is the Tunnel Vision sturdy enough to withstand the vibration from a four stroke? Which YS or Saito would you use? If a four stroke is a bad idea for this plane, what engine would you recommend?

I have already been warned that if I leave the factory covering on it, wearing a feather boa while flying will be a requirement.

Fife, I have one with a 160. At first I thought it was marginal on power, But after Jerry Smith and I experimented with props I'm really happy with it now.
I got a heavy one @13lb and with my setup it does pretty good. First off, you need a Jerry smith pipe! That alone gave me 900 rpm. I found the MAS 18-6 woody got the job done best @8900 rpm, a close second was APC 18-6, similar rpm but a touch less pullout, Pro Zinger 18-6 spins up to 9100 but was marginal on pullout. The 160 sips fuel and I'm running 5% so it's pretty cheap to fly.
If you want to go 4 stroke I'm sure the Saito 220 would do a fine job if you can afford to put fuel in it And I'm sure the airframe would handle it.
I thought about putting my rail mount BME 50 on it but I dont think the nose would last too long.
Your really going to like the way it flies. The roll rate isnt anything like a .40 size profile but is very good for an 80 inch bird. Ive never flown an easier plane to hover, I initially had a little wing rock in upright harrier in the wind, but believe it or not it went away when I switched props, The high alpha knife edge is incredible, and it will pinwheel, something I didnt expect from a big plane.
I passed the tranny around in Nashville, and every one really liked it, Gary Seelof said it hung up a little in roll in rolling harriers, and I think Swany said he would like a little more elevator. But everyone that flew it was comfy enough with it to get down on the deck right away.
Heres a pic of Johnny Mojo flying it

The YS 160 and Saito 220 are the two 4-Strokes that I have in mind. I don't have the specs in front of me, but if memory serves, the YS is lighter. Which one do you think would help get the CG right?

CG wont be a concern. as long as you dont run anything lighter than the os 160. I had to put the rudder servo in the canopy area and the reciever pack is in the fuse right behind the motor. mine balances 8 1/4" behind the leading edge at the root and flys great there.

One more question and I'll leave you alone.

I noticed that there is a factory cut out for a 16oz Dubro tank. Would going to a tank in the 20+ size just be a matter of strapping it to the fuse, or would it be better to stick to a tank that will fit in the depression?

I made a hatch cover for mine and mounted the fuel tank on the outside. I mostly did this to put my rx pack in the fuse