Microwave Oscillator Expertise Desired

[Engineer Tom]

I have a friend who is interested in using a commercially available inexpensive oscillator in the 850 to 1020 MHz range. The unit has a square wave output. He wants to convert the square wave output to a clean sine wave output. His efforts so far have been (expectedly) met with challenges related to stray inductance and capacitance when building an outboard amplifier and resonant circuit to achieve this goal. Breadboarding in this frequency range is different than at audio frequencies.

I believe the solution can best be crafted by someone with experience in this area. He was frustrated by not having the mathematical background, however I do believe practice will trump (let's not go there!) theory in this case.

Does anyone here have such experience, able to assist or know someone who is able to assist him? I have no involvement in this project other than posting this message. I will pass along any preferred contact information so he can directly pursue the matter with the individual.

[AF6WL]

Easy place to start is with an off the shelf filter  https://www.minicircuits.com/pdfs/LFCN-1000+.pdf
Same company has lots of other building block components  e.g. simple to use MMIC amps if you need more output.