If he wants to ride at a title meeting he should look for a 2 Valver, but the rules for the slider class will no doubt change again in near future.
The 2 Valve engines are obviusly getting harder to find and are also fetching more dollars.
Barring that an upright 4 Valve engined bike, Weslake, Jawa or a Godden will be a good start, prefferably in a Hagon, Elstar or Godden frame, or even a Longtrack Jawa, good luck on that one.
Did your mate enjoy the Speedway/Dirttrack at HBB?