Is there any fixing rotting wooden boat interiors, and if so what should I use
Welcome to iBoats . . .
More words and even pictures will help with responses. Are you meaning to say by 'interior' seat bases, frames, and paneling?
If so . . . then . . . Wood or HDPE (aka starboard, marine board, etc). If you use wood there is an 'old timers wood sealant' formula that is frequently discussed on this forum. Use of a good wood sealer will help the new stuff last a long time.
Newer boats tend to use HDPE boards (expensive)