Yes, you need fuses.
The basics of the electrical on your bike are: battery is hooked up to supply power to the ignition, electric starter, and the “alternator” (where it gets a resupply of power once running). These circuits are activated by the key switch. Fuses are generally oriented to protect and separate the components from one another, while the fuse protects the wiring to those circuits.
Simply replacing your battery with a lithium unit is not an involved process, but should involve other considerations, like the Regulator/Rectifier (alternator in your car). The stock unit is not well-suited for lithium batteries so replacement is advised.
If you are as nascent with electrical as this post intimates, you might be better off reaching out locally in your network to obtain some hands-on help, unless you intend to restore the electrical to “stock” condition with “stock” components. Otherwise, a heap of splicing, soldering, crimping, and routing is in your future.
For a general understanding, attached is a very simplified diagram of minimum wiring requirements for your bike.