The substitution of a FET (such as MPF102) for a bipolar transistor in a receiver front-end was a common modification in 1970s.
Heathkit HW-8 : various Internet resourceshttp://home.frognet.net/~mcfadden/wd8rif/hw8.htm
Heathkit HW-8 Pagehttp://www.nostalgickitscentral.com/heath/mods/hw8/hw8.html
Heathkit HW-8 Assembly Manualhttp://www.utm.edu/staff/leeb/hw8.pdf
IF you can obtain a copy of Mike Bryce's (WB8VGE) HW-8 Handbook, this is a compilation if the various modifications suggested for the Heathkit HW-8 QRP transceiver.
All four HW-8 Handbooks now on one CD! http://www.qrparci.org/toystore/thetoystore.htm
The first edition from 1985 by Fred Bonavita, W5QJM.
The second through fourth ( 1986, 1991 and 2003 ) by Michael Bryce, WB8VGE.
Everything you ever wanted to know about the HW-7, 8, or 9 but were afraid to ask!