You've gotten excellent answers, so let me add a couple of more details.

Drilling the holes in the mounts can be a nightmare. To mount the engine strongly enough, you need to use bolts that are a close fit through the holes in the engine's flanges. And then the bolt holes through the mount should give a tight fit as well. What that results in, are holes throught the mounts that're very accurately aligned with the holes in the engine flanges. And since we're working with very hard stuff (bolts, flanges, mounts), any misalignment makes it real hard to get the 3rd and 4th bolt through the holes after the 1st two are placed, unless all 4 holes are very closely aligned.

If you try to clamp your engine to the mounts and use it's holes for a drilling jig, you run into two problems. It's really difficult to clamp that sucker to those oddly shaped, and suddenly very slippery mounts. And if you do accomplish that, a power drill can really trash the holes in the engine flanges. Not good. Very not good! And most drills can't work close to the engine and go squarely through the holes anyway. Thank goodness.

So you're stuck having to mark the centers on the mounts. Some engines come with templates for the job. Kewl if you got the template. HOWEVER.... measure it to see if it's a good match with your engine before you use it. And if you don't have one.......

You find out soon enough that it's hard to keep the engine steady to mark the holes. Use a touch of any kind of glue that works. Glue the engine to the mounts, then mark the holes. And now you find out that a fat pencil won't get through the engine holes and reach the mount. And then you find a thinner pencil and it still won't square up on the side of the engine with that exhaust port sticking out. So you find an even thinner pencil (I've got one from 1970something, I've saved and used for only this job.) and discover it won't mark the motor mount material. So you pull the insides out of a ball point pen.

And discover that it's still fairly hard to get a nice round mark in the center of each hole. Those exhaust ports are a real pain.

But you finally get marks where they ought to be. Now you discover another problem. If you don't get all 4 holes to go through the mount at very close to 90degrees, the bolting down will still be a pain.

I've got a drill press. I've got to be honest and suggest that the primary reason I bought it was to drill motor mounts squarely. And after I bought it, I discovered that most motor mount don't fit easily into a drill press vice!!! And one day, I was rebuilding my front porch railings and had some 4x4 post scrap. And it was accurately square all around. I took the time to set it up to bolt my most frequently used motor mounts to it. And now I use it to hold any mounts that I wish to drill on my drill press. I keep that block of wood with that pencil.

There is an excellent little tool that's sold in most good LHSs. It's a "hole center marking tool". It actually has a small drill as it's marker. And it's got a tapered piece that centers the thing in an engine's bolt hole. You just have to make sure it's straight up for it to accurately mark. It now sits with my block of wood and pencil.