Here is the AMA CL General rules on profiles. I also include a graphic of the fuse width. Notice how CL PA isn't included in the table at all, so to first order, anything goes!. However as Mark points out , all politics, umm I mean profile rules are local, so if you want to particularly fly in P40-like profile events (a non-rulebook event), you might want to check around you. I think a relatively safe version of a P40 "legal" profile is to keep the engine under a 40, and width on the order of 3/4", except at the nose where AFAIK, a doubler or even tripler is routinely allowed.
Also I don't think profile is even defined in the current PA rules section.
so from CLG-6 page of CL-general rulebook (2009-10)
10. Profile Definition. The fuselage of a profile
model resembles that of a conventional airplane
in the side (profile) view and appears as a thin
flat sheet in the plan (top) view. When a
conventional, single cylinder, internal
combustion engine is used, the engine shall be
completely exposed from the centerline of the
crankshaft to the top of the cylinder head when
viewed perpendicular to both the crankshaft
centerline and the cylinder centerline. In the
case of internal combustion engine with multiple
cylinders, the preceding rule shall apply to all
cylinders with allowances made by the officials
for appropriate mounting of the engine. No
fairing may be added to the engine which
violates the ―completely exposed‖ requirement
of the previous sentences, and the engine
cylinder shall not be contoured to present a
streamlined cross section to the airflow.
Additional reinforcements such as plywood nose
doublers and cheek cowls or fairings are
permitted within the width limits defined in the
accompanying table. Such additional fuselage
reinforcements may extend from the prop drive
washer to a point 25 percent of the wing root
chord back of the wing leading edge at the root
and may be faired in. Cheek cowls used in
Racing events may be of unlimited width, but
may be used only on the side opposite the engine
and only on models with side-mounted engines.
In the case of inverted or upright engine
installation, engine mounts may protrude from
the fuselage sides beyond the width limitations
of the table. They may extend no further aft than
25 percent of the wing root chord back of the
leading edge at the root. Any such protruding
engine mounts shall be of constant cross
sectional shape and dimensions, without
tapering. The rear portion of those mounts shall
terminate in an angle of at least 45 degrees to the
longitudinal axis of the fuselage unless they
terminate within the wing structure. Nacelles, as
used on multi-engine models are subject to the
requirements of this definition.