Hi Ken,
Welcome back to CL flying.
You said you have a OS 46 to use but didn't mention what type or vintage. It could make a big difference. For instance an OS LA46 will happily fly up to about a 54 oz airplane but a newer converted OS 46AX (double ball bearing) will fly a 60+ oz airplane with ease.
If what you have is an LA 46 I would strongly recommend (in spite of what fantasy you might hear) that you limit the size to somewhere around 600 to 620 sq inches and the weight to something below about 54 oz.
The 46 LA is happy with either a APC 12.25-3.75 or my personal favorite the APC 11.5-4. Most of the more modern engines like the LA or AX OS's are a little higher tuned than what you're used to in the past and will typically want to run a little higher RPM and somewhat flatter pitch props to develop the proper power range. The newer AX would be happy with a 12 to 12.5-4 or even a a 12-4 three blade!
If you intend to eventually be competitive again in the expert class you'll find it's a bit different game than it used to be, Believe me I've lived and survived through the transition and it takes a little different approach and lots of work to be competitive now! Eventually you'll want to consider either a tuned pipe 60 to 75 size engine or an electric setup. Personally I'm a bit Anti-Electric but admit that they have a lot of advantages in stunt operation!
I understand your desire to design your own airplane but might suggest that you look into a Paul Walker design called the "Impact" or the Randy Smith design called the SV11. They will show you modern numbers and layout that definitely work. As for what they look like...I'm sure you know that small changes in appearance are easy to make while keeping the basic aerodynamics. You'll see things like "pipe tunnels", longer moments, larger stab and elevators than used in the past!
Actually the SV11 is available from Randy Smith as an ARF or ARC. No appearance points but might be a good place to start a return with a very good flying modern stunter!
Best of luck on your return!
Randy Cuberly
PS: I don't want to scare you but in the "tough areas of the country competitive expert scores are in the 580's or higher! The very top guys most often fly close to the 600 mark!