replacement of engine

As I continue to work on restoring a 1974 AMF lawn tractor, I am thinking I am going to replace the engine with a new one. The existing engine is an 8 hp briggs and stratton, I would like to replace with a 17.5 hp briggs and stratton. I've never done this before, any problems with replacing with a higher hp engine? I already think I am going to have to make an adapter plate because the four holes to mount the motor to the body of the tractor are not the same. Also, the existing engine had wiring going to a solenoid and to the starter switch, but I believe this new motor will have a solenoid already installed? Just not sure how to wire the new motor compared to how the old one was wired. Any suggestions appreciated. This is the motor I want to replace with: http://www.ebay.com/itm/Briggs-Stratton-Engine-31R907-0007-G1-17-5-hp-Intek-repl-31C707-3026-/321353996908

Just some thoughts: A new 17.5 Briggs single cylinder is overhead valve and is about 2-1/2" longer than the old flathead 11 hp, which was longer than your existing 8hp.Uusually that is only a problem if there is not enough space between the grille/hood and the engine. I bet some website online will have a dimensional drawing of the engine.

The big thing is the diameter and length of the crankshaft. It has to match your old engine or else belts don't line up, and pulleys don't fit.

The Big difference electrically is your new engine will probably have a fuel solenoid. (Unless its some really rare spec. number-I do recall seeing one late model 12 hp without a fuel solenoid.) You will have to figure some way to power it from your existing wiring harness.
The other electrical difference is that the new engine will probably use the common 6-pin plug. (But only have 4 circuits through it though.)You can do away with the plug. Use the charging stator from the old engine and it's plug, (You may have to use the old stator anyway, so it will work with your chassis wiring harness. Some of them had an AC circuit for lights.) Use the kill wire to the coil from the old engine and its plug, and that would only leave you to deal with the fuel solenoid in some way.

Thanks. Sorry for silly questions but this is new ground for me. My existing engine has a frame mounted solenoid, power from the battery goes to the solenoid and then the other solenoid terminal goes to the starter. Then there is a third spot on the solenoid which goes to the key switch I believe. In terms of the new motor, yes it has the six pin harness. So if I bought a new key switch how many plugs would it need on the back? And if I cut off the six pin harness, I guess I have to trace the wires to find out where they go? I believe you are correct that the new motor has a fuel solenoid. Maybe I should take some pictures of the old set up and post them, I don't see a stator but I could be missing it. I have many hours I into this restoration already, every nut and bolt has come off. I have the main tractor deck stripped and painted, all of the moving parts in the undercarriage have been stripped, painted, and reinstalled, the transaxle has been reinstalled, wheels stripped/painted, new tires. One rear wheel had to be cut off and replaced, it was seized solid. I'm now working on stripping and repainting the front end, once reinstalled I will have a rolling chassis again. Still a long way to go before putting engine back in, but when I do it doesn't make sense to put an under-powered 43 year old leaky engine back in after all that work.

I've attached a photo of how the current motor is wired, shows the starter, the key switch, and the solenoid. Any thoughts on how to make this work with the six pin harness in a new motor would be appreciated.

Sorry photo didn't work, here is a link: http://s1284.photobucket.com/user/TMan159/media/wiring1_zpsnqow9evp.jpg.html

(quoted from post at 08:34:01 07/06/17) The existing engine had wiring going to a solenoid and to the starter switch, but I believe this new motor will have a solenoid already installed? Just not sure how to wire the new motor compared to how the old one was wired.

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That Briggs engine will not have a solenoid attached to the starter. It will need a stand alone solenoid like the old L heads.

You will need to plan to do some extra wiring for the Carbs fuel shut off valve and Charging system.