PowerShell is an option, and one I would go for if this will be a recurring need.

Here is an option using some T-SQL and the SSMS GUI scripting features that could get you there pretty fast:

1. Backup your database and restore it to a test instance.

2. Run this SQL on the restored database to get rid of objects you will not be modifying.

DECLARE @sql NVARCHAR(MAX);

SET @sql = N'';

SELECT @sql = @sql + 'DROP PROC ' + QUOTENAME(s.name) + '.' + QUOTENAME(p.name) + ';

'

FROM sys.sql_modules sm

JOIN sys.procedures p ON sm.object_id = p.object_id

JOIN sys.schemas s ON p.schema_id = s.schema_id

WHERE sm.definition NOT LIKE '%string your looking for%'

AND p.is_ms_shipped = 0;

PRINT @sql;

-- uncomment when ready to do work

--EXEC(@sql);

3. Script all remaining procs as ALTER using the Object Explorer (Press F7 in SSMS).

edit: add "AND p.is_ms_shipped = 0" to query