Ruger,
That 61 shouldn't be a problem, in fact I think that's a better choice (especially since you already have it). The fuel tank will not be a factor, the airframe won't know the difference you can pull just as many G's with a 46 as a 61, beef up the fire wall with a little epoxy, move the battery back in the fuse and you might not have to add any weight.
I have a Flip and it calls for a 2 stroke 46 and I got a OS 91 4 stroke hangin out there!! No problem at all. I beefed up the firewall, moved the bat back and works great. I'm gettin about 10-12 mins out of it (stock fuel tank).
Besides your not adding that much weight and the 61 and 46 are similar in power out put, besides 4 strokes sound cooler anyway!!! JMHO