That thing'll beat the crap out of the fire glue joints (well, ALL the glue joints) and the all-up weight will go WAY up due to the engine and all the lead you'll need to balance it.

[ul][*] Weight also means it'll stall at a higher speed, which will make landing runs longer and much faster. [*] Weight will also ruin it's very nice, low, wing loading. [*] You'll need to shoe-horn a bigger tank in there but since a Dubro 16oz is already the width of the fuse you'll have to find one that's longer.[*] Oh, and more fuel means EVEN MORE WEIGHT...[/ul]
In conclusion:
DOn't worry though, it's not a bad idea...It's a a horrible one!
Listen to your safety officer!
You will LOVE the way it flies with the FS120.