The 4-40 screws are plenty. Lots of clamping force.  Four to five in-lbs results in around 240 lbs clamping force per screw as a first approximation.

These are usually called: Screw, Cap, Allen Recess .112-40UNC-3A. That just means .112 nominal OD; 40 threads per inch, which the reciprocal gives .025" per thread; Unified National Standard (60 degree threadform with root radius mods); the class fit is 3, which means not too tight or too loose for easy assembly but good strength; "A" signifies that we are talking about a male thread (bolt or screw) not a "B" female thread (nut)

Of course, I am assuming that we are talking about an engine that has not had the holes drilled out. Stock, they should be about .131" ID.

If you wanted to, you could use the 3mm screws shown in the OS owner's instruction manual. But they are only .006 inches (about two sheets of paper thickness) bigger in diameter. Your choice.

I prefer to do the most accurate job possible locating the holes in the engine bearers, including any engine offset I plan to use. Then I don't have to oversize the holes in the engine to try to get the thrustline trimmed in. Also, when there is a pattern of holes that go together, a machine shop will be given a "true position" tolerance, meaning how much each screw can be off, based on some math and geometry. As you probably intuit, the bigger the clearance holes in the engine lugs, the more tolerance can be alloted to the positional error when you drill the hole in the engine bearer. These numbers are directly related--or else it may not go together. The good news is that the OS tolerances for position error are going to be very small, so you can use up nearly all of the assembly clearance for drilling your holes in the bearers, or for some slop so you can shift the engine around a tiny bit for spinner fit (uniform gap) or thrustline adjustments.

You are using the bolts to clamp the engine lug down to the mount and the preload creates sufficient friction to avoid slipping. Bolts are not used to carry shear loads. Pins are used for that when it counts. So pins use a very close fit to the holes on both sides of the joint, often times even a press fit on one or both sides. But we don't need that on model engines.

Something to keep in mind is that when you torque a screw down, you are both stretching the screw and compressing the stack of materials between the screw head and the nut. (Equal and opposite forces.) If you are using wood bearers, the stiffness of the joint is dominated by the much less stiff wood. (Lower elastic modulus than the steel screw.) So, if you want a stronger installation with less loosening, etc. fix the bearer design, don't keep upsizing the screws!

A good way to stiffen up a wood joint and keep things from progressively loosening is to thread and glue in a steel or titanium threaded insert into your bearers. This is done on some racing models and works very well--but it is extra work. And, with good eastern maple or birch engine mounts you don't risk too much crushing if you keep the torque to reasonable numbers.

Generally, you want the nut to be of a softer material than the screw so that it micro-yields to cause load-sharing between all of the engaged threads. Remember, the bolt is stretching, and you don't want only the very first thread in the nut to carry all the load--because it won't. In a practical sense, any hardware you are going to buy has all of this engineering already baked into it. You do want a minimum of around five threads engaged to carry the load reliably. If everything was perfect (yada, yada, yada), then three threads would develop enough strength, but....

During my younger years I always used 4-40's to mount almost everything (#2-56 or #3-48 for 1/2A) because that's what was available in Hobby Shops and some hardware stores and it worked fine. Of course, that's when there were numerous local hobby shops and most of the engines I was using were made in the USA. Fast forward to later years or to current times and that's no longer the case. I fly a lot of F2D combat planes and some High performance 1/2A and they are all drilled for 3mm mounting bolts and use 3mm or 2.5mm screws in the engine (some have no screws in the engine). Most of the items we use come from Ukraine , Russia, or European distributors and can't be sourced at a local shop. In fact local hobby shops are almost extinct or don't carry u-control items. The aluminum motor mounts we use are also tapped for 3mm bolts so I now use almost all metric hardware. Here are a couple of very good sources for metric (and US) fasteners. They stock everything you need for hardware; Socket screws, nuts, locknuts, washers, Blind nuts (t-nuts), etc.

https://www.boltdepot.com/catalog.aspx?gclid=CKulkdia0dECFQ9rfgodmgECdw

https://www.fastener-express.com/

Thank you Motorman👍