Radioisotope thermal rockets would be excellent for my small drones and seem easy enough to calculate, maybe qswitched can throw them in someday or they can be modded in if modding support eventually allows new modules.
Radioisotope thermal rockets would require large radiators to deal with the heat when the rocket wasn't running. And even if you were willing to have them burn only once and then eject the core, the mothership would have to store the fuel in a cooling pool (again requiring radiators) and insert it right before launching the drone.
If using a high melting point rtg fuel such as op related, I think I could use the engine bell itself as a radiator. These drones of mine are only 6.2 kg, the engine doesn't need to be all that powerful and I could make the engine bell larger for more efficiency and more radiator space if needed.
Increasing temperature through oxides or ceramic impregnation or a liquid containment for a longer lived isotope is probably the more fruitful approach.
I revisited this recently -- it looks like the most promising compound with Polonium would be rare earth polonides. TmPo has a melting point of 2200 C, density of 10.28 g/cm^3, and YbPo has a melting point of 2212 C, density of 9.08 g/cm^3. But finding this out already required me library access ("Rare Earth Polonides"; C. J. Kershner, R. J. Desando, R. F. Heidelberg, R. H. Steinmey; J. inorg, nucl. Chem.. 1966. Vol. 28. pp. 1581 - 1588) -- and I couldn't find the other required properties.