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The first thing you need to do is make sure your tractor is wired for negative ground. Alternators are negative ground while generators usually run positive ground. You can swap the battery and amp meter leads and usually be done with it because the starter will turn the same way regardless. You will have to add a balast resistor to the coil if you don't swap it for a 12v coil. Concerning which alternator.... I had a similar situation on my MF-2135 and found a Nissan alternator worked perfectly. It was almost the same diameter as the generator and bolted up with no modification. What you are looking for is an alternator with two additional terminals (usually spade lugs) along with the main output terminal that runs to the battery. These two terminals are the exciter and sense line. Mount the alternator and connect the output lug to the battery or amp meter and put two short leads to the other terminals. With the engine running, touch each of the two terminals to the alternator output lug. One will draw a very small spark... that's the exciter lead. The other won't draw any spark because the sense lead is very high impedence. Tie the sense lead to the output lug and forget it. Connect the exciter lead to your ignition switch. I usually tie it to the (+) side of the coil. That usually is all you need but occasionally, some alternators will backfeed voltage out the exciter lead. If that's the case with the alternator you end up with, your tractor will keep running after you turn the key off. Just shut the gas line off and let it die to kill it. Then, add a diode (Radio shack has them) that can handle several amps and put it inline with the coil wire to the exciter line. Diodes have a line painted at one end to identify the cathode end. Wire that end to the coil side and the other end to the alternator and you will block any backfeed from the alternator to the ignition. Drop me an email if you need any additional tips or pictures.
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