Hello and Welcome aboard!
Ill try to help you out the best I can, ask me anything that ya need to know that I didnt cover,
Is this motor for a mini or a kart? Is it going to be raced seriously, or just like a pit bike?
Im going to go out on a limb and say that None of the parts will work, (easily)
The connecting rod has the chance of being too short, (the briggs numbers do not match)
You could have a cam ground but that is going to cost some serious money to have done (100-300 dollars)
Valve springs wouldnt be too big of a deal, as you could find heavier ones, but lifters would be a custom,
Id leave the stock valves, even my raptor has stock valves.
A better carb (True Mikuni, Tillotson, Walbro) would help the most, along with a true header. and loosing the governor, (risk of destroying a motor)
In all honesty if your looking for the most power out of a motor, without busting your bank I would look into the 5 horsepower, (Yes, there is no true substitute for cubic inches) but for the dollar for dollar it may be much cheaper ( briggs 5 horses can easily have 15 horsepower, and as high as 40+) The 8 horse just doesnt have the mass of parts that the 5 does.
Be aware that lightening the crank can have ill effects, and you could possibly severely weaken it, especially under high RPM loads.
Im not sure how much you are willing to spend, but speeds gunna cost cash, Not saying it wouldn't be worth it, or be fun but it WILL be costly.