Yongboom, my corrections and suggestins are typed in following each of your sentences:

1. When it comes to pizza, America has good restaurants than Korea.  better restaurants
--> Regarding pizza, America has good restaurants than Korea.   regarding is fine; better restaurants
2. Our company is one of the best company when it comes to a field of car suspension.  'the' field of car suspension    also, when you say 'one of the best' you must say 'companies', so, it is one of the best companies

--> Our company is one of the best company regarding a field of car suspension.   regarding 'the' field of car support

3. He is a famous person when it comes to dealing with some issues for our clients  some issues OF our clients
--> He is a famous person regarding dealing with some issues for our clients  regarding is fine, but 'of our clients' 
4. I am enthusiastic when it comes to(regarding) food.  Both okay, except I would suggest that you put 'when it comes to' at the start of the sentences:  "When it comes to foed, I am enthusiastic."  Regarding is fine in the middle of the sentence, as you wrote it. 

Best regards, Lucky 

Thank you!