Hearthpwn

I don't have a Doomhammer but I have Al'Akir the Windlord. Would that be a viable replacement?

Great deck, any updates on this with the last wing released?

Fel Reaver Mech Shaman is a really strong, fun and overlooked deck. I would def replace a Flametongue Totem with an Earth Shock or two though. It really helps you counter cards like Sludge Belcher so you can continue pounding the face. :-)

Great deck.  I won 6 in a row in casual mode and then did 3 wins/2 losses at rank 9 on the first evening.  Fel Reaver does seem to be a bit tricky, as there are some opponents with low curve decks and good card draw.  I find it bad to play vs. paladin (Aldor Peacekeeper) and priest (mills like crazy).

Having 2x Crackle and a Lava Burst really helps close out the close games when you need to go face over a sea of taunts.

I've been playing a very similar deck, minus 1 Fel Reaver and plus 1 Earth Shock. I'm considering cutting something for another Earth Shock actually -- Sludge Belchers can destroy this deck.

I also run Windfury instead of Flametongue Totem. Really powerful (and unexpected!) source of burst. Here's a link to my deck: http://www.hearthpwn.com/decks/237474-windfury-aggromech

any replacement for fel reaver

I've thought about trying Clockwork Giant as a replacement, but I haven't actually tried it. Same body as Fel Reaver, but it's rare that you'll be able to play it on turn 5. Your opponent would need to have 7 cards in hand, or 6 if you have a Mechwarper out. It might be worth trying, but the fact that you have to wait for your opponent's hand to fill up not only makes it hard to play consistently, it also increases the chance that they'll have removal at the ready.

Spectral knights. kinda

Fel Reaver works great in the deck because it's 8/8 for 5 mana and the only downside is that it burns cards you'd probably never see anyway.

You could try Sea Giant, which is similar size and can often be played at ~6 mana cost, but it's not as reliably pulled out early.  The other stuff that has similar stats at 5 mana are Earth Elemental and Venture Co. Mercenary, but they have side effects that could stall your momentum going into the next turn.  There's not much else that has that amount of threat at that cost.  I think a decent budget card replacement would be Boulderfist Ogre, slightly slower and smaller, 6 mana for a 6/7.

Edit:  Just tried Drakonid Crushers in the deck, in place of the Fel Reavers.  They work well.

Maybe Piloted Sky Golem? Its not exactly the same and it costs 1 mana more, but its another big mech you can put in play that impacts the board.