Replacement parts

I got this bike and a Haro bike from a customer of mine, his kids out grew them, and I'm looking for decent part to replace them. I replace the inner tubes and tires myself. Pegs/handle bar grips/brakes are the what I'm trying to replace. Doing it myself so any advice would appreciated. I'm doing this for my 2 kids. Their a perfect age for these bikes. All their friends ride electric bikes but if I can customize it with their wants I figured they would ride them. I use to have a GT Dyno and a GT Performer. Dyno didn't last me 3 months before it was stolen. Sorry for the long post. I'll see my way out. Thanks in advance from a 80's kid