I've done mine three times over the years - actually 4 times because I was off one cog and had a "do-over". It really is not difficult - all engines are just a bunch of nuts and bolts. I found it MUCH easier to lift the engine into the cockpit and then hug it properly, but I've got a Santorin and only about 8 inches of clearance on the belt end of the engine and I don't function very well upside down - it may be ok to do in-situ on your SM if there's more space to work.
No special tools required. I use a cheap "pop" type tension meter and you can supplement that with a reasonable "eyeball" of the tension. I used drill bits to lock the flywheel and camshaft although $pecial steel pins are available. Shop manual says to also lock the flywheel at the starter motor, but that's redundant with the pin in the flywheel and is just for babies - duh, disconnect the battery so nobody tries to start it.
They do sell kits with new idler and tension pulleys, but just a new belt should be sufficient, imho, and my shop manual doesn't suggest replacing the pulleys. Most important part, I found, is to carefully count the cogs between the cam shaft and fuel pump pulleys, mark them with nail polish on the pulleys and the belt. If you are one cog off it will run, but with a severe case of dyspepsia.
It's a really fun project - you'll get some bragging rights, too!