Agreed,old spa is a fantastic track,and a new spa from s397 would be awesome. I personally think that all the tracks (and cars)that come with Rfactor 2 should be updated,at the moment the sim feels a bit broken ... and a new Targa would be a dream !!
There is an AC conversion that seems pretty much complete and works well but its not at the level of quality that S397 builds. The 66 Belgium version is fabulous for just about any of the cars I run. I'm with you on the McLaren GTR at Belgium 66, very tunable car for that track once you turn off the cruise control options.
Time to post my most loved racing related video again Sir Jackie Stewart (i'am a big fan) explains how to drive Monaco in a F1: We have everything in rF2 to experience in sim what he is talking about (1966 Monaco, 1966 F1 and great physics).