Richard,
The YS 1.40 is the recommended engine. There is even a heard and exhaust that is supposedly for the plane that doesnt require any cutting for clearance. However, a buddy with that setup DID have to cut the cowl a bit.
There are other less expensive options than the YS. A Saito 1.8 or 1.5. OS and Moki make 2C engines that are a good fit as well. The MDS 1.48 is a great little engine for the money. Mine runs VERY well and has beaucoup gallons of fuel thru it (I think it's starting to loosen internally......)
Also, make sure you put some right thrust in as well. We had a little aerobatic school up here last year and Jason Shulman came up to teach. When he flew mine his first words were "RIGHT THRUST!!!" I ended up with two washers under the my engine mount! the spinner isn't centered anymore but it SURE flies well!