You are quite correct in saying that materials and designs have come a long way, you only have to look at what Darren has to say about his engine, designed and built like a brick s**thouse and will go on for lifetimes, but only at a certain rev range and power output. Modern designs are another concept, being more like a four stroke engine rather than the heavy cast iron jobs from times gone by.
I can't comment on the modern stuff as I have had very little to do with them, but in my day, the large model diesel was the holy grail for modelling enthusiasts.
There is a massive difference between the reciprocating weights and forces of four 5cc engines stuck together, or two large 10cc units trying to work in unison. The bigger you went, the more strength had to be built in, and more problems trying to raise the compression ratio to get a good detonation and burn. If I was going to try it, I would go down the route of the Taplin twin, small cylinders, but more of 'em. Because they are basically a two stroke, it is very easy to put multiple cylinders along a common crank. To me, an 8 cylinder 20cc engine will be easier to achieve than a twin cylinder 20cc.
I am sure that using modern designs and materials, then your ideas might come to fruition, but I am sure most of it will be frustration and burning the midnight oil to achieve your final goal.
You have to remember, they might be available in the outside world, but the manufacturers have money to throw at projects such as that, because they have a final market and profit. You will be working by yourself, with the only prize of a massive grin when you eventually get it going, and most probably a write up in a model mag. The big boys were most probably at your stage 20 years ago, so it might be to your advantage to see if any of those can give you any pointers along the way. There must be a method of getting over the large cylinder/bad combustion phase. On the engines that I worked with, they were very low tech, maybe you should be looking for high tech solutions, to get you thru the barrier.
Bogs