OS 91FX(?) w/ 15x5 will give you about 15lbs static thrust(Usable / Realistic). So your 540 needs to weigh in at less then 13lbs (Complete).

OS91FX
Displacement: 0.912 cu in (15.0cc)
Bore: 1.091 in (27.7mm)
Stroke: 0.976 in (24.8mm)
Practical RPM: 2,000-16,000
Output: 2.8 bhp (ps) @ 15,000 rpm
Weight: 19.3 oz (550g)
Includes: Muffler, Safety Propeller Locknut Assembly, #8 glow plug
Requires: fuel, mount & prop
Recommended Propellers: 13x8-9

Personaly id go with a 15x4. The 15x5 seems to overprop it a bit.