does it run? If so, then the flywheel key isn't "broken"...
What problem is the engine having??
To replace a flywheel key on a walker, yes, the engine must be pulled out, but it's not that big of a job, 1/2 an hour labor at the MOST. Replacing a flywheel key is a 5 minute job and 10 cents in materials, then another 30 minutes to reinstall the engine. A decent shop would charge you for probably 1.5 hours of shop time. Any good redneck would fix it in his own garage, lol.