Recovering a specific job definition from an MSDB backup?

No need to restore to a scratch server, you can restore it to something like you say (MSDB_old) and do a query to get your job back:

USE msdb_old
SELECT * FROM sysjobs
JOIN sysjobsteps ON sysjobs.job_id=sysjobsteps.job_id
WHERE sysjobs.NAME='My Lost Job'
ORDER BY sysjobsteps.step_id

You'll have to restore

• the entry in sysjobs
• each entry in sysjobsteps related to the above entry in sysjobs
• entries in sysjobhistory if you want history back
• entries in sysjobschedules to get your schedules back

EDIT: Here's a script that should do it in SQL 2005 and 2008 (assuming your job was called "My Lost Job" and you restored to MSDB_Old)

DECLARE @JobID UNIQUEIDENTIFIER
SELECT @JobID = job_id FROM msdb_old.dbo.sysjobs WHERE NAME='My Lost Job'

INSERT msdb.dbo.sysjobs
SELECT * FROM msdb_old.dbo.sysjobs
WHERE job_id=@JobID

INSERT msdb.dbo.sysjobsteps
SELECT * FROM msdb_old.dbo.sysjobsteps
WHERE job_id=@JobID

SET IDENTITY_INSERT msdb.dbo.sysjobhistory ON
INSERT msdb.dbo.sysjobhistory
    (instance_id,job_id,step_id,step_name,sql_message_id,sql_severity,
     [message],run_status,run_date,run_time,run_duration,operator_id_emailed,
     operator_id_netsent,operator_id_paged,retries_attempted,[server])
SELECT
    instance_id,job_id,step_id,step_name,sql_message_id,sql_severity,
    [message],run_status,run_date,run_time,run_duration,operator_id_emailed,
    operator_id_netsent,operator_id_paged,retries_attempted,[server]
FROM msdb_old.dbo.sysjobhistory
WHERE job_id=@JobID
SET IDENTITY_INSERT msdb.dbo.sysjobhistory OFF

INSERT msdb.dbo.sysjobschedules
SELECT * FROM msdb_old.dbo.sysjobschedules
WHERE job_id=@JobID

The script is loosely based on this one in the accepted answer. It has been updated for SQL 2014, with exception handling, atomic transactions, and a few other improvements.

-- Script for SQL 2014
DECLARE @JobID UNIQUEIDENTIFIER
declare @servername sysname

set @servername = @@SERVERNAME

SELECT @JobID = job_id 
FROM msdb_old.dbo.sysjobs 
WHERE name='My Lost Job'

BEGIN TRAN

BEGIN TRY

INSERT msdb.dbo.sysjobs
SELECT * FROM msdb_old.dbo.sysjobs
WHERE job_id=@JobID

INSERT msdb.dbo.sysjobsteps
SELECT * FROM msdb_old.dbo.sysjobsteps
WHERE job_id=@JobID

SET IDENTITY_INSERT msdb.dbo.sysjobhistory ON
INSERT msdb.dbo.sysjobhistory
 (instance_id,job_id,step_id,step_name,sql_message_id,sql_severity,
 [message],run_status,run_date,run_time,run_duration,operator_id_emailed,
 operator_id_netsent,operator_id_paged,retries_attempted,[server])
SELECT
 instance_id,job_id,step_id,step_name,sql_message_id,sql_severity,
 [message],run_status,run_date,run_time,run_duration,operator_id_emailed,
 operator_id_netsent,operator_id_paged,retries_attempted,[server]
FROM msdb_old.dbo.sysjobhistory
WHERE job_id=@JobID
SET IDENTITY_INSERT msdb.dbo.sysjobhistory OFF

-- New insert in sysschedules 
SET IDENTITY_INSERT msdb.dbo.sysschedules ON
INSERT msdb.dbo.sysschedules (schedule_id, schedule_uid,
         originating_server_id, name, owner_sid, enabled,
         freq_type,freq_interval, freq_subday_type,
         freq_subday_interval, freq_relative_interval,
         freq_recurrence_factor, active_start_date, 
         active_end_date, active_start_time, active_end_time,
         date_created, date_modified, version_number)
SELECT schedule_id, schedule_uid, originating_server_id, name,
       owner_sid, enabled, freq_type, freq_interval, freq_subday_type,
       freq_subday_interval, freq_relative_interval,
       freq_recurrence_factor, active_start_date, active_end_date,
       active_start_time, active_end_time, date_created, date_modified,
       version_number 
FROM msdb_old.dbo.sysschedules a
WHERE schedule_id = (select schedule_id from msdb_old.dbo.sysjobschedules b where job_id=@JobID )
SET IDENTITY_INSERT msdb.dbo.sysschedules OFF

INSERT msdb.dbo.sysjobschedules
SELECT * FROM msdb_old.dbo.sysjobschedules
WHERE job_id=@JobID

-- Alter job as local job
EXEC msdb.dbo.sp_add_jobserver @job_id=@JobID, @server_name = @servername

END TRY
BEGIN CATCH
SELECT
    ERROR_NUMBER() AS ErrorNumber,
    ERROR_SEVERITY() AS ErrorSeverity,
    ERROR_STATE() AS ErrorState,
    ERROR_PROCEDURE() AS ErrorProcedure,
    ERROR_LINE() AS ErrorLine,
    ERROR_MESSAGE() AS ErrorMessage;

    ROLLBACK TRAN

    RETURN
END CATCH

COMMIT TRAN

The easiest way to extract a single job from MSDB is to right-click the job in SSMS and say Script Job - then take the script to the target server and run it to recreate the job (with potentially some modifications needed).

This only works if the msdb is restored as msdb - which means in your case you'd have to restore the backup as msdb on a scratch server.

I guess you could restore it as a copy of msdb and then manually pull everything out of the various msdb_copy.dbo.sysjobs/sysjobsteps/sysjobschedules/sysjobservers tables using a join.

Hope this helps!