You can get an alternator with tach drive from this site. Problem is, the only one they offer requires an external regulator. No big deal, just that the regulator is mechanical, something else to go wrong. You will need to do some wiring modifications, and fab a mount. It is essential the alternator be mounted with the pulley properly aligned.
As others said, keeping it 6 volt has it's advantages. However, the 6v system will only work if everything is right and the engine is properly tuned for easy starting.
If your charging system is working, it would be easier to just fix the starter and keep it 6 volt. The starter may only need bushings and brushes, something that can easily be done by a starter/generator shop. A common mistake found on 6v systems is the cables have been replaced with automotive cables. Automotive cables are much too small. A 6v system needs size 0 or 00 cables to carry the high amperage. Also the original ground strap connected to the sheet metal behind the battery. A better ground is connecting the cable directly to the engine block.
I would stay completely away from the 8 volt option. Little to gain, the regulator would have to be adjusted to gain anything at all. 8v batteries are slow movers, likely the one you buy has been on the shelf for years.