Yep that's the bracket. I guess it's something you could fabricate if you wanted to otherwise I can send one. Similarly I can send you a cranked bolt or you can make one. Dimensions aren't super critical. Length is such that the head sits slightly higher that the pulley (by about a 1/4" or so). It's attached with two nuts- one above the deck and one below. Probably should be a spring washer under each nut as well. You want a minimum of excess thread sticking through on the underside of the deck when you're finished, of course. Bolt diameter is 3/8", I think but confirm with the size of the hole in the deck. Crank in the bolt is such that it will prevent the belt coming out of the pulley when the cutter clutch is disengaged.
Also, in your collection of bits you have a piece of flat bar 1" wide and about 10 or 12 inches long with a fairly big hole in each end. You need 4 of these. These are the bars that form the parallelogram ( one on each side ) that support the deck. The large holes in the bars (1/2" or 9/16" diameter) are bushed with loose bushes with an ID the size of the studs in the deck mount brackets. I guess these are 3/8". Bushes are slightly thicker than the thickness of the bars such that the nuts can be nipped down firmly and still allow the bars to pivot. There should be wave washers here too but to be honest I don't think they do a great deal and I'd be happy to leave them out.
