You have all the parts- no need for another clutch unless the driven takes a wider or narrower belt- there's ways around that too! Here's the link for a belt:
http://www.parts-unlimited.com/home/...436E51F4A22087 (they sell them by measurements.)
Your motor/clutch/driven don't care how long or short the belt is!!You'll need to do a little searching to see what the longest/shortest belt with the right width you can get to know what your limitations are.
I much rather have direct adjustment in my chain vs. a chain tensioner(another maintenance item). The diameter of your axle sprocket needs to be known-(likely 50t-60t) that tells you where the jackshaft position is for the shortest chain length. Chain tensioners need space!! so if it's a really short chain-NO tensioner.The shorter the chain the more accurate the alignment of sprockets too. This should help get you thinking about what & where to go next!!!
How long are the threads on the engine adjuster bolt?That's your limit of adjustment.