As there are no lights or horn, it sounds like no earth or poor earth connection to the battery. The horn usually has it's own connection direct to the battery due to it's relatively high consumption. The lights are wired through the ammeter and main switch - so, as neither work, it is most likely an earth fault. Also, if there is an earth fault, with the engine running no charge will show on the ammeter, but the dynamo may be giving off enough power to run the lights. As an initial check, connect a bulb across the battery and see if it lights. If it does, connect it to the live side of the battery and any earth point on the engine or frame - if it lights, the battery earth is OK and the fault lies elsewhere. If the bulb does not light at all, the battery could be dead and not giving enough power to excite the dynamo into producing any charge. Try substituting the battery, but before starting the engine, momentarily bridge the live side of the battery to the 'F' terminal on the dynamo - this will polarise the dynamo to suit the polarity of the battery (after laying idle for some time, dynamos often loose their residual magnetism, so dont produce enough power to charge the battery).
Hope this helps,
Regards,
Eddie.