Is there any ballast other than a magnetic 20W Mh ballast (without ignitor)and a 50W M.V ballast that would correctly power the 50W MV lamp? I have a 50W mv i would like to use!

Any ballast that closely matches the lamp current will work, without the ignitor. You just have to look it up. I'm thinking a 35w mh ballast, but I'm not sure. I'm sure somebody will reply with more info.

I read on LG from Dave's pic and on yahoo groups that you can use a LPF single lamp 40W preheat ballast to run 50W MV lamps, I didn't try this myself but I've read that it seems to run fine.

F40T8 ballast would deliver 0.43A instead of 0.6A, so the lamp would be ~28% underpowered assume all involved things are in the middle of their tolerance range. It might still light, but the performance and lifetime (blackening rate) would be gone.
If you are talking about 230V serial choke, using small CFL choke (8..13WT5 or PL-S 5..11W; add ~0.16A) in parallel would correct the current to about 0.58..0.6A, what is what the lamp need. Unfortunatelly for 120V area the thing is not as easy, as you might in such way combine only some ballasts and it is difficult to tell, what ballast use what phenomenon (L or R or C) for bsallasting.

@20WMH: As far as i know manufacturers require special electronic ballast for these lamps, what is able to activelly stabilize the arctube temperature, what will not work with MV.

Use a 50w MH ballast. They are electrically the same (90v at full brightness). Just remove the ignitor.

Use a 50w MH ballast. They are electrically the same (90v at full brightness). Just remove the ignitor.

Dave, are you sure that would not overpower it by 20-30 watts? I will try it as soon as i get the ballast, actually I should just get a 35W MH ballast to be safe.  How would I go about wiring the lamp to the preheat ballast?

  How would I go about wiring the lamp to the preheat ballast?

Hey Aaron I've linked a wiring diagram for you in my comment on this picture on LG.

Use a 50w MH ballast. They are electrically the same (90v at full brightness). Just remove the ignitor.

Ballast is a current source (the current change only a little with the load voltage), so you should compare the current and not the voltage at first place.
They are not the same: 50W MH is 0.7A (so has about 77V arc), 50W MV 0.6A (~95V arc), so the lamp would be ~16% overdriven.
Note, then on the market are lamps, that are designed to actually draw more then rated power by using higher voltage arc (e.g. "50MH" nominal is actually 60W input power lamp with ~95V arc voltage). This cheat is used very often by inferior manufacturers or on otherwise disadvantaged lamps to "catch on" lumen output of their better "skilled" competitors (e.g. Rx7s "70W" QMH are often actually 75 or even 80 watters to compensate for lower efficacy of horizontally burning arctube).

I should just get a 35W MH ballast to be safe.

MH35W is 0.5A for ~80V arc, so would yield ~0.48A for 95V arc (MV50W), so the lamp would run at ~40W.

^Then that should work just fine since 50w mercury lamps are designed to run at 40 or 50 watts.