A dynamo is a permanent magnet, AC generator, you have one under the fan pulley, the AC needs to be rectified to DC via a rectifier, then controlled via a regulator, (depending on model), or by a combination rectifier/regulator.
Permanent magnet dynamos have low output, so replacement higher output alternators are sold to fit many kubotas and generally mount where the belt tension pulley are currently located. Check to see if your current dynamo and regulator works first before buying an alternator, unless you are really going to need extra power output.
As this motor has been around a long time, personally I would replace the bearings in the fan/dynamo while you are at it. Probably 2 or 3 6201's or similar, do a search and the correct bearing size will have been mentioned previously in another thread I expect. Also check out the tension pulley bearing, but if you could leave that one until you prove your dynamo etc., in case you want to fit an alternator.