Multiple line search and replace bash sed

I have a file called tst.txt that contains text:

exec(compile(open(bla bla bla'))
DATE = "05JUN20"
Only capture the below line
params = {'System_Date': getparamDate(DATE), 'Param': '2'}
This Line should be available
''End

'Short description:
'bla bla bli, bla bla bla

First line with }
Normal Line without bracket
Another bracket}&
Then normal text
blabla 
blublu 
tatati 

I want to capture and change only the line:

params = {'System_Date': getparamDate(DATE), 'Param': '2'}

in my file.

I've fond a sed command that allow to substitute what is between patterns. /c

sed -i '/params = {/,/}/c params = MyNewParam' tst.txt

where I intend to substitute only what is between:

params = {

and

} from the same line.

unfortunately, my regex will replace until First line with }

exec(compile(open(bla bla bla'))
DATE = "05JUN20"
Only capture the below line
params = MyNewParam
Normal Line without bracket
Another bracket}&
Then normal text
blabla 
blublu 
tatati 
I5TaxActivationDate

which is not what I expect.

I've seen that with

,/}/q

it's possible to stop at the first occurrence, I cannot find the right syntax.

Can anyone help please?

Thanks in advance

Using GNU sed

Capture and return with back-reference \1 params up to and including {' within the parenthesis.
Exclude everything up to } and replace with MyNewParam

$ sed -Ez "s/(params[^{]*\{')[^}]*/MyNewParam'/" input_file
exec(compile(open(bla bla bla'))
DATE = "05JUN20"
Only capture the below line
params = {'MyNewParam'}
This Line should be available
''End

'Short description:
'bla bla bli, bla bla bla

First line with }
Normal Line without bracket
Another bracket}&
Then normal text
blabla
blublu
tatati