I have few zenoah +1mm crankshafts need to be repaired and Some of the crankshaft seem to be quite loose. Can't find a good way even i 've try to weld it ( photos enclosed). Can you please tell me What is the best way to repair the twisted/bent crankshaft? Please show a few photos of it. Thank you.

One of the best discussion I ever read on this topic is over on JRCBD. The
thread is titled "Best way to check cranks".

This thread includes:

Mounting cranks using Shaft Centers vs V-Blocks
Measuring crank deviations with dial indicator(s)
Repairing twisted cranks by replacing the Crankshaft Pin (see JA's posts)

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Welding Cranks: Just from what I have read, some can make welding work and
some cannot. Since welding is hit or miss (until someone posts a precise
explanation of how they do it consistently), I would prefer to try JA's pin
replacement method.

Look at all the pluses for pin replacement:
- if the assembly misses the mark, it can be redone
- no welding slag to worry with - one tiny slag chip coming loose inside the
assembled engine may spell serious trouble
- no issues with excess welding heat
- can often result in a stronger crank than any OEM

Welding is not the preferred method to prevent crankshaft twisting because the crankshaft cannot be used again. Consider the fact that the only part of any crankshaft that can wear is the crankpin itself. Therefore, the replacement of the crankpin in a crankshaft material of the correct metallurgy, with a crankpin hole that is ground & burnished can be done many times without losing the press fit amount. With the correct crankshaft metallurgy, press fit amounts of .0013" will prevent twisting of the crank pin. Properly designed fixtures, jigs & a 12 ton press are some of necessary tools.

Bending of the crankshaft can be prevented with better metallurgy & of course by increasing the standard 12 mm (.4274") diameter to 15 mm (.5906"). This change can be effected by replacing the standard bearing with a 61902 size bearing. No modification to the existing crankcase bore diameters will be necessary.

Jim Allen