Oil restriction
 

Just a quick one. Is oil restriction a good idea in a race motor, when going from a hydraulic to solid cam ? How do you do it if you are not using a special lifter ?

Yes, but the first step is to ask the manufacturer of your lifters whether they are designed to internally perform adequate oil restriction. If so, then you only need to verify that the lifters' oil bands are in the right place- each lifter boss oil feed hole should be blocked when the lifter is on the base circle of the cam.
If not "all of the above", then you'll need to add restrictors- either tap the feed holes for restrictors (1/4" plugs with .030" holes) or find restricted-style pushrods.