Your 20dB insertion loss at peak is far too high. Something is wrong with your interdigital filter, probably too wide spacing between the coupled lines. These oscillators in particular were built hundreds of times, mostly worked from the first try. The author optimized the circuit and the layout for the specified transistor, so if you changed something, like replacing the original BFP183 with BFP450, it's no wonder you get a worse performance. In addition, I agree, silver coating will not make any difference. S parameters and the datasheets for these transistors should be more than available...