In my opinion....... If you put an 8 hp tec on there, you just ruined the kart. To put that engine, you gotta drill new mounting slots. (8hp on ONE tire??? Really?) You said tire preservation...... That 8horse is gonna eat that one tire up in a couple weeks. Those scrub brakes belong on that kart. They work great, ihave some on a kart. Stops on a dime, if needed.....
To do all the stuff you said, is just, well, not right in my book. You're taking your dad's childhood ride and gonna butcher it all up to what today's standards are..... You might as well buy you're kid a cheap yard kart then.
If you REALLY wanted your kid to have something, get that engine from Russ! Put that kart back into its former glory. No other kid around will have one, it will be fast (when YOU want it to be) with that awesome 2 stroke sound, and the best part.........
It will be as your dad had. You're fathers childhood excitement, and sense of adventure will be reignited within your son.
For the love of God , man...... Don't change a thing...... If you truly just sit back and think it through, you will see that I'm right. What do you really think you're dad would have wanted for the young lad? I bet it would have thrilled him to know his grandson was gonna be tooling around on a kart he drove as a child..... Your father would get to relive his childhood, through your son......
Dude, if I stepped on toes, hurt your feelings, or was just out of line...... I apologize. I thought about this kart all night and the special circumstances that put it into your sons grasps.... The history behind it all. And then I wake up and read thatbyoure going to hack it up........

That truly troubles my heart. You're lucky to have what you have. Don't screw it up now. (please) I'd pay out that wazoo, to have something like that....... Just for the sentimental value.....
Speaking of value...... For every alteration / modification you do, it drops in value.