Hi, Firstly to deal with your last question first,
No, the Heng Long board (RX18) is not really able to be modified, although the latest RX18 is a world apart from the early versions.
Taigen's similar latest Control board is a massive advance which at last gives you the option to use the Taigen Transmitter OR an after market transmitter with different sound modules available, It is probably the cheapest aftermarket Control Board available.
There are a multitude of aftermarket Control Boards available, all of which require a separate receiver and Transmitter to operate, ALTHOUGH whichever aftermarket control board you choose the same aftermarket Transmitter /Receiver will work with it unlike the Heng Long Transmitter which is exclusive to Heng Long Tanks,
Here follows some Web sites---
http://www.rctanksaustralia.com
This Australian Dealer sells and makes some truly innovative products for our hobby, amongst them is the "ASP2" which for its small size and low price is a very good board indeed,
It is very close to being as much a "Plug and Play" unit as the Heng Long RX18 but with a far better sound card and all round abilities,
Next ----
http://www.rctankelectronics.com
This company offers a very wide range of Control Boards which includes the IBU2/the IBU2 PRO/the IBU 2u Base/the IBU2u Full option/the IBU 3. These are much used in Forum Members Tanks and "Ian" is the main man and is a member of this Forum and will answer all your questions.
The IBU3 really needs you to have the use of a laptop or PC to get it up and running whereas the IBU2 of all types makes changing sounds easier with access to a lap top or PC but not vital.
Next----
http://www.elmod.eu
makes control Boards for a wide range of R/C vehicles, the "Fusion" range is the one for Armour.
Next----
http://www.clark-model.com
This Company was one of the first in the aftermarket Control Board field with the likes of the TK22 through to the TK60,
The TK22 fits into the same holder as the RX18 but is far better in every way,
The sounds of the main gun loading, firing and unloading is a standard facility included within each control board I have listed as is an engine noise unique to each tank ,
Each control board can be programmed with different/extra sounds.
Every aftermarket control board is fine in a Tiger 1 and most of them use the Tiger 1 engine as the default engine sound,
You are of course free to change that engine sound for the one most suited to your Tank including the Challenger and Abrams.
I would suggest you go to each site and read up on what they offer.
Your Forum has a section devoted to aftermarket control boards under the "Electronics" Heading it is worth reading the many contributions.
Shaun.