LO power, at least in a passive mixer, wants to be pretty high to drive the mixer. So putting in a large value attenuator just to "match" the amp to the lo port sounds counterproductive. Can you place the amplifier physically right at the LO port, so there will be no standing wave with a lengthy transmission line between the two. That way you will get ok power transfer and no big ripples in the LO power vs. frequency.
if not that, then:
at low frequencies (< 1 GHz) you can probably just use a wirewound transformer.
At higher frequencies, maybe a quarterwave transformer?