Honestly before spending a lot of $$$ on upgrading the deck, you might want to look into replacing it. The price of various R2R machines has dropped to rediculously low levels... look at getting an Otari 5050 (it can switch between 2 track and 4 track playback but can only record as 2 track) or a Studer PR-99 (newer version of the B-77). If you look hard enough, you can find one of these for about $500... the cost to upgrade the Studer could very easily approach this.
Know anybody who mods analog r2r tape machines?
I've obtained an old REVOX/STUDER B77 MK II together with some Jazz and classical tapes at a garage sale. The sound is amazingly good with a nicely layered soundstage. However the sound is also typically old solidstate, a bit harsh and grainy. Here in its country of origin no tech is interested anymore in this old stuff and wasn't the US in the old days called the "country of infinite possibilities" to those who dreamt of leaving the old continent? Wonder if this still is so....