For stats click a pic or the links below. Or visit the only places I found them, WoB or Deathshadow.
These are the ones I know about and have stats for. There are rumours of a Fire Moth and a Kit Fox as well. Most lack Endo-Steel (which saves more tonnage!), and most are at the heavier end (lights need ES and FF). Some have unassigned armor and free tonnage (probably due to forgotten heat sinks). One thing that really sets them apart is the attention to detail. Most of the time, they have heat sinks in the legs, ammunition in the arms and a weapon load that is heat-balanced (so that you have "replacement" short range weapons at the same heat cost). Also, the design of the miniatures reflects the Mk 1 configs and not the released ones, which I'm sure you've all noticed. They are, simply, the answer to all our questions...
NB: I don't actually know who wrote up these designs, but they are supposed to be from a magazine, and I've found them in various different places, so I think they are accepted as the "real" mk 1s. Thanks to World of Battletech and Deathshadow (wherever he is) for getting them on-line, and to James McKenna for preserving most of Deathshadow's work.
Nova: Has neither ES or FF. The Nova is fairly slow for a 50-ton clan mech, especially with a close-range primary variant. The Mk1 is 6/9/6 and otherwise identical due to ES and FF. This makes it a much better Clan medium, but it doesn't do much to separate it from the Stormcrow, out of which you can make all the Nova variants. This may be a reason they changed it, too.
Stormcrow: Nothing to fix here, necessarily, but the prime has too many HS. The Mk1 reflects the look of the mini, with the SRMs above the cockpit, in two different variants, which I really like.
Mad Dog: The Mk2 lacks ES, but has a greater weapons load than the Mk1. The difference is speed. This is maybe the most radical change of a mechs personality between Mk1 and Mk2, and I'm still undecided on it. It changes from being a bruising fire support to a mobile fire support, and we already have the Linebacker.
Hellbringer: This is the major reason to do this. Adding ES and FF lets you add tons of armor, which it badly needs. It then becomes an absolute killer and no mistake, a true successor to the Wahammer.
Summoner: Add ES and you gain those missile pods on the minis legs, but nothing major. Solid.
Timberwolf: The Mk2 retains ES and FF, so nothing to improve. In fact, the Mk1 is worse, somehow. A lot of the Mk 1 loadouts feature LRM-15s with Artemis IV rather than LRM-20s, and this is one of them. .
Gargoyle: Helped by ES, the Gargoyle becomes an assault mech at last, with added armor and weapons.
Warhawk: The Mk1 adds ES, allowing it to get heat under control (mostly by ditching the 4 PPCs). Primary variant looks just like the mini, with a SRM-6 beside its cockpit and 4 ER Large Lasers. Deadly.
Executioner: ES allows the Prime to carry 2 PPCs instead of 2 ER Large Lasers. Enough said.