Good "heliarc" welder can do it at welding shop! If the head warps, - it can be machined (with possible increase in compression), and of course un-even-ness can be filed down!
I have removed small warps or severe scratches in past with a piece of sandpaper taped on a piece of glass! -- you just move head around in sort of a figure "8" path, and the corners will wear down to a "medium" surface somewhere between the two,-- or if just severely "bent" on one corner, you can try to file it down till it is fairly near "level" with the rest of the head, --then even out with sandpaper putting more perssure on that corner it it is near even, then matching whole thing by sanding with "even pressure" on whole head! Just take your time, and check often to see how close it is getting to "level"! You will be able to tell, as the "workled area" will be different than the un-worked area-- if you wipe your finger across it a couple times after checking -- this sets up "pattern marks" for next try!... If a large amount is removed, you may want to use washers to take up difference, - if it looks like the bolts will "bottom out"!
I have done this with outboard motor heads, and B&S engine heads before, -- also on mounting flanges on cast iron and aluminum manifolds -- both flange on manifolds (or pipes), and flages on heads!