Nope, you just need 7.4v and 3.3v. The batteries output 7.4v which will power your screen as well as the 12v line on the N64. It doesn't actually need 12v; whatever you supply will be stepped down to 5v (supposedly you can remove the regulator and supply 3.3v to the 12v line, but that's a totally different topic). As for the 3.3v line, head to the N64 megasticky and look at "supplying 3.3v to your N64." You simply need a small regulator from TI that will step down the 7.4v from your battery to 3.3v.
So, to recap:
Battery 7.4v --> screen
Battery 7.4v --> N64 12v line
Battery 7.4v --> TI Reg 3.3v --> N64 3.3v line
WARNING: DO NOT supply anything greater than 3.3v to the 3.3v line and DO NOT bridge the 12v and 3.3v lines at all costs. This will FRY your motherboard, guaranteed.