How would one perform a more thorough preflight of the engine on a single-engine airplane without easy access to engine compartment?
These issues don't pop up that often that you would need to include them on a preflight check. Those aircraft/engines are reliable enough that any issue can usually be determined by things like:


Excessive oil use
Oil Temperature / Pressure Changes
Rough running or improper behavior during run-up


That is why you do a run-up in these aircraft that checks many systems, so you can spot those kinds of issues. But the reality is that even if there is an oil leak or missing safety wire, those issues are reviewed (in detail) and fixed in the annual inspection. The aircraft is reliable enough to not worry about it between inspection intervals. For rental/hire aircraft this inspection happens more often, every 100 hours. 

Some of the issues may be caught during oil changes as well, which happens every 50 hours (depending on manufacturer recommendations).

Let me give you an example...

I was doing some recurrent training and selected a Piper Cherokee. I was doing the preflight and you can open the cowl on those aircraft. I noticed a significant oil leak behind the controllable pitch mechanism which my instructor and I decided not to fly until the mechanic looked at it. When the mechanic did, he said that it probably needs to be rebuilt but to go ahead and fly it anyway, just make sure that the oil was full prior to flight.

On my 177 we had been flying it all year, when it went into annual it was found that the piston controlling the constant speed propeller was way out of spec and that the hub had corrosion. The entire prop had to be replaced. We were flying it and it was behaving just fine, but could have failed at any time.

The point is that the issues that will stop your engine from running or make your prop run away are usually not visible anyway. Keep it topped up with oil and pay attention during run-up and you should be fine. Usually I take a flashlight and peer around in the compartment as best I can to make sure nothing has taken up residence in the engine, but aside from that, there isn't much else to do preflight.