Shell Programming and Scripting

Hi Gurus,
I have below requirement and have no idea how to achieve this.
the input file like below. there are multiple sections in file, each section has multiple lines. I need to find certain lines (value1, value2, value3 are key words for line searching) and generate another file. in some section, maybe some lines missing. for missing line, output "Missing"
My OS is SunOS 5.10 Generic_150400-64 sun4v sparc sun4v

Code:

[section_abc]
xxxxx
xxxx
CITY=ABC
REGION=CDE
STREET=EFG
[section_cde]
xxxxx
xxxx
CITY=xyz
REGION=123
STREET=345
[section_efg]
xxxxx
xxxx
CITY=900
REGION=200
[section_ghi]
xxxxx
xxxx
REGION=500
STREET=600

expected output file like below:

Code:

[section_abc] CITY=ABC REGION=CDE STREET=EFG
[section_cde] CITY=xyz REGION=123 STREET=345
[section_efg] CITY=900 REGION=200 MISSING 
[section_ghi] missing    REGION=500 STREET=600

thanks in advance