The holes are in an awkward position and getting it right is quite different than most rethreading. The only easy part is that it is aluminum. if a retainer is off I would really be surprized if the valve isn't bent. That of course will change everything.

The tools necessary to do valve guide oil seals are what you will need to reinstall the retainer which is the first thing I would do. To do this requires holding the valves up with air pressure through the spark plug whole. If you can pressurize the cylinder the valves will probably be all right. If you get air out the exhaust pipe - listen for it, the head needs to come off. If that is the case fixing the bolt holes will probably be easier with it off.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician