Well, it looks like IMDB is a bit incomplete when it comes to that other guy who played Jason in Part 6, as nothing about Jason Lives is listed on his filmography whereas everything from Joy Ride to Spider Man 3 to the Bourne movies is there.
You mean in terms of official credits I assume, because Tom Morga (who also played Jason and Roy under the hockey mask in "A New Beginning" along with Dick Warlock who, of course, played Michael Myers primarily in Halloween II before returning as a robot whose face was itself a mask in Season of the Witch. BTW, apparently due to his recognition of the significance of such props, Dick Warlock...acquired both the Friday The 13th Parts 3, 4, and 5 Jason Voorhees hockey mask AND the original Don Post Shatner Michael Myers mask after wearing them in the movies) and his firing are covered fairly extensively in the documentary on the making of Halloween 4 on the Anchor Bay/Scream Factory Deluxe Edition Collection bonus disc and the NSM Region B Blu-ray version.
