Yes, I replaced the old headlights on my roundtail with new H4 units without any difficulty. Something like CL04F on this page: Classic Car Accessories HEADLIGHTS
, it was a drop in replacement. The previous owner had fitted halogen versions of the old style bulbs into regular lights but proper H4 units were much better.
HOWEVER, I would strongly urge you to upgrade the wiring for the headlamps at the same time. I don't know about the S2 but on my car the earth was via several bits of bumper before it got back to a decent chassis ground and the full load was going through the fragile column switch. It isn't too difficult to add proper earths and make up an extension to the wiring loom for a set of relays (I posted a picture of mine a while ago if you search for it). This will benefit you in even more light and reduce the likelihood of having to replace the light switch.