Nowadays you don't to be smart at all, aside from your search engine queries.
Google search results are poor - its focus is more on shopping, based on their profile of you. Use other search engines.
You went to electronic parts sites that are about selling you the part... but it's very old and their databases will be useless for that.
Some of what I'll do, for the fun:
Look at the IM-2420 Heathkit docs, BoM and schematic -> Q1 is an N-ch JFET source-follower, good to 500MHz and DC-coupled into Q2.
Shows Heathkit p/n 417-828 with footnote "selected 417-802". Original is "E304" I
DSS 5-15mA.
Lookup the freq. counter's history:
https://archive.org/search?query=IM-2420&sin=TXTThe IM-2420 announced 1980 for $249 kit, $300 assembled. Kit almost $900 in today's dollars.
Same front end as the IM-2410 but that is biased to 3mA.
Gets lost in old magazines.
Search for "E304 JFET". This is a transistor from 45 years ago so Digi-Key won't list those, the ones that were not popular.
Also I found Q2 417-293 is 2N5770 RF NPN 30V 50mA fT=600MHz TO-92 see PN918, maybe MPS918 as well.
https://www.d8apro.com/https://www.qsl.net/w6ovp/heathkitparts.htm