These little bikes are pretty hot.You won't need to change engine parts to keep up with most other british bikes.Just put it together properly,and massage a few parts in the process.
The cams are equivalent to 3134 cams,but in the short stroke engine they work 1000 rpm higher.You can change cams without splitting the cases.You don't even need to remove the head from the barrel to do it.You need to lift the barrel enough to fit re-ground tappets,if you change cams.Cut the base gasket in 1/2 (if you use one) along the frame centreline,and join it back with Loctite 518.
The tappets are "R" radius,but lighter than 650/750 tappets and have a cup to fit the pushrod aluminium end (no steel cup at the bottom of the pushrod).
There was a similar topic at Britbike recently :http://www.britbike.com/forums/ubbthreads.php?ubb=showflat&Number=387793
Do a perfect valve job. Leave the seats in the head 3/32" wide and 0.020"-0.030" smaller diameter that the O.D. of the valve-head and back-grind the intake valves at 35 degrees to get 1/16" wide contact.
Make sure you have at least 0.002" intake guide clearance (right through the guide,especially at the centre),and 0.0025" exhaust guide clearance.
You can lighten the top valve spring collars a little,by squaring them.It doesn't hurt to grind some weight off the extreme ends of the rockers,around the adjuster and the pushrod ball.You don't want valve-float at 9000 rpm.Measure the fitted (seated) valve spring load with scales.
Don't be tempted to use big exhaust pipes.The 1-1/4 pipe is about right at 6000 rpm. 1-3/8" would be OK when you get to 7500 rpm.
12" intake length (from valve to bell-mouth) is good at 7500 rpm and also gives a torque boost a little over 5500.If it's shorter,you need more rpm to make it work.
If the exhaust length came in at 6500,you'd have a happy engine.
Weigh the pistons and make them the same.Radius any sharp corners around the valve-pockets and polish the crowns,1/8" radius where possible.
If there are any scratches/nicks in the conrods,polish them out vertically with #180 grit paper.Hold a rag underneath to catch any grit.
Check the cam timing,but don't bother with 0.020" tappet lift figures.Check it at 0.080" lift or so,and calculate the lobe centre,before you change anything.You can adjust it in 4.8 degree increments,using alternative camwheel keyways.
There must be something I've forgotten,but it's mostly just careful assembly.