At the Pica web site, under "builders corner", there is a picture of a 1/6 scale T28 with a OS 90 4stroke. The engine doesn't appear to stick out at all.
Bruce mentioned the Saito 80. I looked at that engine to. I would check it out. That engine is smaller and if you read the specs in the manuals, the hp rating isn't that different than the 90. I think that would be a sweet engine.