Okay, I've raced Ariel's since 1977 on and off,so first rule : begin simply with stock parts and develop it.
I began with VH 500 1949 and in mind to race in Vintage sidecar class,so steel flywheels if you plan to rev them which has different parallel pin and cage to iron wheels (taper ).I then raised the oil level in the camchest drilling a 3/16" hole to the left of basecircle of cam and then get a short piece of 3/16" bundy tube and put slight S' bend into it so it goes from drainhole in timing cover into the old crankcase drainhole adjacent to the crank pinion,this takes any unwanted oil from timing cover and stops magneto drowning in oil etc,buy extra tube as you may not get it right first time,remember smooth bends and seall with Hylomar at each end. If you have a post-war pump in good order fine ,pre-war change it and change to R40,it's the only oil that takes hammer from hot iron engines,also I ran mine on Methanol,but if you only plan short sprints etc not yet necessary. I ran initially with B50 genuine BSA stock piston to up compression. Bear in mind overboring weakens barrel which is only base-studded, I have an 88mm 2-valve Jawa pistoned barrel but that's thru' bolted,a lot of work. I initially used standard valves but then went to JAP inlet,a little larger and ideally once you've done a season go for something with 5/16" stem for better flow,leave that standard exhaust valve in there,nos is best as I've experienced pattern fusion-welded valves where head drops in and costly,soif it attracts a magnet forget it. Cam was standard single lobe A cam to begin and then reground to HS timing,some people also add Mk3 followers. Springs initially were standard softies 422? I think, Do not fit the Terry's so-called HS springs , they'll wipe out your cam and levers in no time. When you get it reliable and change valves put W+S Goldstar multi-rate springs in with top collars and cotters,you'll need dural bottom packers and JAP insulators ,oh and a pen and paper. Carb size try 1 3/16". Just remember reliable finisher,hope this helps,T