There is many options available, it depends what you want. Vape is probably the most popular. They offer an A/C or D/C version. If it were my bike, I would buy the 12 Volt D/C kit. You would have to change out the 6 volt components - the light bulbs/flasher - and install 12 volt bulbs/flasher. The 6 volt horn will still work, without having to replace it. It will be louder.

Vape: https://www.vape.eu/en/yamaha-dt-250360400dc-version

Rex Speed Shop: https://rexs-speedshop.com/

HPI: https://www.hpi.be/