flap - I have a 150 sized engine in my 120 sized Edge 540 - if you read ahead of all these posts, you'll get answers to most of your questions.
Suggest you stick to a 90 sized engine if thats what size arf you got. Another guy put a 120 in the 90 sized plane and was not too pleased, but like i said, read ahead!
Cheers
tfacr