Rich, check the following:
http://home.pacbell.net/fraze/replicraft.html
Their plans are 1/5th scale drawings with full scale dimensions. I know of 3 full-scale SE5a's being built using these plans. Yes, you can find Hisso engines. Perhaps you can contact me offline about that. I don't have full information but can get suggestions. If you want a full scale aircraft it will take several years. I don't know the cost, but you could amortize it over those several years and it wouldn't be too bad. It would also depend on how many of the fittings you make yourself and how many you have to have made by an outside source. Big difference in costs there.
If you don't have the time and patience (and $) for full scale the following offers a 7/8 scale aircraft which uses a modern powerplant.
http://www.historicair.com/replicaplans/
I've sat in the cockpit of both. This one is quite small feeling but I think goes together more quickly.
Personally I lean towards full-scale but it's more of a commitment as well. Stick around this website and
www.Canvasfalcons.com and you can learn a lot. Also find places that restore old aircraft and make friends. You'll learn even more.