No you can't. The bonnet (the top bit) is an integral part of the reg body. If you remove that, all the small springs and stuff will fall out and it might even leak
The on/off ASA idea would solve the length problem, but it won't cure a leaky pin valve, a common problem on preset air regs (at least the usual type of on/off, which has a moving pin that opens and closes the tank's own valve.)