Top-ranked North Dakota State and third-ranked Central Oklahoma will receive their toughest tests of the young season when they compete at the Cliff Keen Invitational in Las Vegas on Friday and Saturday.    The meet is expected to draw more than 50 teams, including a number of top Division I programs.    Fourth-ranked Michigan, No. 8 Lehigh, No. 9 Illinois, No. 10 Arizona State, No. 11 Nebraska, No. 12 Edinboro, No. 13 Fresno State are among the Division I teams scheduled to compete.    North Dakota State has a pair of top-ranked wrestlers in defending national champions Todd Fuller (174) and Nick Severson (heavyweight). Seven Bison wrestlers are ranked in the top eight in the country.    Central Oklahoma also has a pair of top-ranked wrestlers in Cole Province (133) and Muhammed Lawal (197). Central Oklahoma's Mark Dodgen, a national champion in 2000 who finished third in 2001, is ranked second at 125 behind defending champ Adam Mars of Adams State.    Two other Division II powers - No. 2 South Dakota State and No. 4 Nebraska-Omaha - are headed to the Northern Iowa Open on Sunday at the UNI-Dome in Cedar Falls, Iowa.    South Dakota State has four wrestlers - Aaron Veskrna (157), Tyler Bryant (165), Tyler Jones (184) and Jon Madsen (heavyweight) - ranked second in the nation.    Nebraska-Omaha is led by second-ranked Chad Wallace (197) and third-ranked Zach Stalder (184).    Fifth-ranked Ashland (Ohio) is scheduled to play host to the Simonson Open.    No. 6 Pitt-Johnstown is slated to wrestle in the Penn State Open on Sunday. Pitt-Johnstown has No. 1-ranked wrestlers in Joe Lloyd (149) and Shad Benton (157).