Hi Bob, the biggest issue with the larger engine is the fuel tank. I'm not sure if you have seen my build pics but I wasn't happy with teh stock tank. The triangle shape of the tank and the way it was shaped to the bottom of the fuse made it impossible for the pickup to work effectively. To get it to travel up and down without hitting the back of the tank meant I had to sacrifice length of the pickup line meaning it would miss a bit of fuel (pic attached). I ended up removing the ducting to fit a new dubro tank (1.8L). This wasn't enough for the VT80 unless I wanted a 4 minute flight and I have since added the modified xcalibur tank which is 2.4L.
Even then because of the steering servo location it can only go so low into the fuse and can only be a certain length as it fouls the equipment tray former. (remember I am only speaking on behalf of the SM version, not sure what the top RC situation is)
If you are using a cockpit you are then also limited by height. If you are not using a cockpit you could get a higher tank in there with a larger capacity. If I was building this again, as it is a 'sport scale' airframe, I would just tint the canopy and lose the cockpit for a larger fuel tank.
Hope this helps.
Jeremy