If there's something to be said of the nostalgia kick lately in movies and TV...and if the popularity of MG among adults I know personally translates to ratings, then I see no problem of going the MG 90 route and repurposing Charles Nelson Reilly. As for current stars, Brad Garett was good on MG 90. Other names that spring to mind as good, funny participants: Jon Cryer, Dave Coulier, Roger Lodge, a number of the "Scrubs" cast, etc. A lot of this depends on what network did it, or if it went into syndie. There are a number of people, thanks to reality TV and the current pop culture landscape, that are popular enough for this show and available.
-Jason