Cruise Control Help Needed

I have a 78-245 DL.

My volvo has a very early version of Volvo factory cruise control not
made (or only partially made by Volvo). This speed control involves a
Dana Speedostat cruse control controller (which has a split odometer
cable so that the Speedostat can "see" the vehicle speed). The
speedostat has some electrical connections and some vacuum connections.
One vacuum connection is the supply and one connection is the control to
the vacuum servo.

After all these years, my cruise control went kaput. The problem: Rubber
in the vacuum servo has deteriorated (ripped and cracked) to the point
that it no longer holds a vacuum. About three months ago, I repaired
(from the outside) a rip in the vacuum servo's "bladder". That repair
worked for about two months. Last Friday I removed the bladder from the
vacuum servo and patched it with a bicycle patch from the inside and
gave the inside a thin layer of RTV all around. I now have my cruise
control back again, but I know it's only a matter of time. Dry cracked
rubber exposed to the heat of the engine compartment cannot last
forever.

Volvo says that the vacuum servo is no longer available. The part number
is 1234724. I have been looking all over for a replacement part. I
might even be able to use a vacuum servo from another vehicle of that
era (or even its bladder). The problem is that the 78 B21F engine did
not have any electronics, so the entire cruise control is analog. The
vacuum servo does not have any built in electronics. Most modern day
cruise controls have integrated electronics.

I am considering several possibilities:

1. Trying to locate the part as old stock at some Volvo dealership.

2. Trying to build a new rubber bladder (somehow).

3. To install some or all of of an aftermarket cruise control. I's like
to keep (at a minimum) the internal cruise control stalk and under dash
electronics as I think that these are almost standard.

One of the problems with this approach is the detection of the speed.
Since I have a manual transmission, I'd like to detect speed from a
spark plug or coil pickup. This works only if you engage Cruise control
in 4 th gear (or 4th + overdrive more likely). The clutch switch drops
out cruise control anyway. Some aftermarket cruise controls only use a
tach input to detect over-revving as a safety feature. Some aftermarket
cruise controls actually let you choose where you get the speed signal
from and include the tach as an option. Others offer magnet kits with
pickups that go onto the drive shaft--I'd like to avoid the extra wiring
and engineering with that. Some kits put a miniature speed transducer
in line with the speedometer--but I'm worried that these are only
appropriate for US cars. Another problem is the mechanical connection to
the throttle cable from a different servo.

Any ideas on any front?