I'm pretty new to the RC flying world but really enjoy it. With that being said... I have a 60 size hangar 9 corsair that I plan on putting a 1.20 in. Is this to much engine for the plane or will it be able to handle the stresses fairly easily?

if its a 60 size plane than use a 60 size motor, max 75, keep the weight down it will fly better

If it is a 1.20 size 4-stroke, then yes. I've also seen this plane with small 20-28cc gasoline engines and it flys very well.

a 120 2stroke definatly over powered for a Hangar-9, the 120 4 stroke is really on the edge of whats needed to fly these lightly built sport warbirds.

if your using a OS engine then a 91 surpass works fine in them for a Saito a 90-100 is plenty of power.

if its a engine you already have you might have to mount the battery in the tail to get it to balance my Hangar-9 planes balanced out perfectly with a OS91 4st.

I would highly recomend doing a search in the warbirds section on the plane there is alot of information and cautions about overpowering the H-9Corsair without strengthening areas like the firewall and wing joints.

I did a search myself and got over 300 results

Here are what my rookie eyes see:
Based on the reputation and general flight characteristics of a warbird (especially with the wing design of the Corsair) I would opt for a light weight four stroke. H9's site did reviews with a Saito 1.00: the fit looks really good with just the tappet covers clearing the cowl. It is substantially lighter than something like an OS 1.20 four stroke, and appears to have plenty of power for sport-scale flying. The plane is not designed to be an all-out aerobat, so I don't see a need for over-powering and weighting down the plane.

I have a Venus II with a 1.20 two stroke, and it weighs just under 9lbs. While this is a totally different style plane, it still has over 110 square inches more wing area than the corsair, and is approximately the same weight. I like the idea of scale planes and scale flying, but if I can shave weight and improve flight characteristics, I will. I flew a Decathlon with a few different setups, one which included some extra on-board weight - it was a lot less fun to fly that way.

I probably have the least amount of experience out of anyone else in this thread so take this with a grain of salt.

I have seen a couple of them at the field and from what I have seen the 1.20 would be over kill even for me. I saw one with the Saito 1.00 and the other with the OS .91 and they seemed about right for the plane. I know Joe, me holding back on the power?? I feel it's more of a weight thing though because it is lightly build and doesn't need the added weight or vibration.

As has been pointed out, you have not specified if you have a 1.2 two stroke or four stroke. It will be a big difference. If its a Saito 1.20 four stroke then the power to weight ratio is not in your favor. The Saito 1.20 is kind of a dog compaired to the new Saito 1.15 that is about the same size as the Saito .91 but puts out like a 1.20. If you have an OS 1.20 ax then it will be rediculous power for the plane but you would not be the first. I recently read a thread where a 1.20 ax was installed in a Hanger 9 60 size Mustang. That being said the Corsair can be a handfull for the inexperienced. I have been flying for over 3 years now including a 50cc Aerowworks Extra 260. I recently picked up a Top Flight 60 size Corsair with a Super Tiger .90 two stroke in it that I hope to maiden today (the previous owner built it and never flew it). My point here is even with my other experiences I am going to aproaching this maiden flight with extream caution due to the Corsair's reputation. Aditionally every Corsair I have ever seen at the feild (two) has taken off, rolled left and planted.