i don't think a そう version is necessary

@@ -15,1 +15,1 @@
-<gloss>suspicious looking</gloss>
+<gloss>suspicious-looking</gloss>

Quoting from 胡散臭い entries:
GG5: うさんくさそうに人を見る look suspiciously at sb
中辞典: うさん臭そうに見る eye [look at] a person suspiciously
ルミナス: 彼は私を*うさん臭げに見た He looked suspiciously at me

To me うさん臭そう looks like a regular formation from 胡散臭い. As an adverb うさん臭そうに applies to the verb; not an object, so it has to be the person who is looking that is acting suspiciously. Possibly the only odd thing is that you don't use うさん臭くに.
I don't know about an entry for うさん臭そう - I'll see what others think.

@@ -6,0 +6,3 @@
+</k_ele>
+<k_ele>
+<keb>うさん臭い</keb>
@@ -14,0 +17,3 @@
+<gloss>questionable</gloss>
+<gloss>dubious</gloss>
+<gloss>fishy</gloss>

Or, maybe the entry should just be for 胡散臭そう, "suspicious".  I just ran across it as 胡散臭そうな目付きをした, which is a な adjectival, but again, the adjective describes "looking with suspicion", rather than "suspicious looking".

As in the example sentence for this entry, there's another nuance of this word, as in 胡散臭そうに見る -- "to look at someone suspiciously".  In this case, the adjectival meaning of "suspicious looking" or "shady" isn't correct.  But I'm not sure whether to add an adverbial meaning here, or if there should be a new entry for 胡散臭そうに, since that seems to be a common adverbial form.