Thanks for the positive rating Greg, I had added some comments to my previous post that are relevant, they didn't make it through our server though.
Hello again, there are a wide range of safety issues here. I thank 'Electrofier' for mentioning the 18" rule for garages.
I was focusing on your question regarding adequate combustion air however..
Beyond that there are the issues of storing flammables in the room now.... or later, and clearance of the wood burner from adjacent walls.
The existing clearance is most likely safe enough but needs a discussion...fire rated wall, ceiling and door construction is another issue we have not addressed.
If possible it will not be a bad idea to put two half sized combustion air grills near the floor, to afford some circulation in the event that some time later, someone stores flammables in the room.
Regarding the 18" clearance, the fire box, particularly the hot ash pan at the bottom can easily be closer to the floor than 18"... that needs to be addressed, and measures taken in that case to isolate the bay with the boiler and wood burner in it from the garage area.
I had assumed there would be a local inspector to look at the job, since there is none, perhaps we should discuss the full range of variables. Let me know if you require that or not.
( My background is in mechanical engineering nationally, and in California as a mechanical contractor, with an unlimited boiler license... 52 years, recently retired.)
If I read you correctly the gas fired boiler is rated at 130,000 BTU's and all we have on the wood burner is its 10" diameter stack. In that case code for the gas fired unit alone is 70 square inches of *open area combustion air intake grill. Then we have to estimate the peak btu rating on the wood burner, judging by its stack diameter... and I will double that to account for over stoking etc.
The wood burner fresh air intake alone should exceed the diameter of the vent stack by 100% ..area of the vent stack is 5 x 5 x 3.14 or a bit under 80" of open grill space x 2, or 170 square inches to be safe for the wood burner alone... then we add 80 or 100 square inches of open air intake for the gas fired water heater...( if it is that you are referring to as 130,000 btu/hr).
I would split that up into two 150 square inch combustion air intake grills... with 1/4" galvanized wire mesh over them and rain hoods over the top.
We need to consider a fire rated door on the bay as well, if the fire box or ash tray on the wood burner is anywhere near the floor.
Let me know what you think, we can go from there. Perhaps the other expert would like to comment on the fire rated walls, doors and ceiling construction as they might apply in this case.