Thisara Perara is fit and firing but hasn't performed for Sri Lanka in ages. In fact he's playing in Pakistan atm and played BBL while SL were in SA. Also Kapugedera shouldn't be in the best side. Kusal Perara or even Gunathelika should be some where there.I'd say this would be something approximating their best T20I squad at the moment, with everyone fit and available:
Niroshan Dickwella
Upul Tharanga
Angelo Mathews (C)
Dinesh Chandimal (+)
Asela Gunaratne
Chamara Kapugedera
Thisara Perera
Dhananjaya de Silva
Nuwan Kulasekara
Lasith Malinga
Lakshan Sandakan
Sandun Weerakkody
Milinda Siriwardana
Dasun Shanaka
Binura Fernando