As you probably know, Isuzu has exited the US market and there is no longer support infrastructure for replacement Isuzu parts. Thus, it is very difficult to find replacement remotes for many Isuzu factory keyless entry applications.
What you may not know is that many of the keyless entry systems used on Isuzu vehicles were supplied by other automotive manufacturers. If you know what you are looking for, you can still find replacement remotes for the systems supplied by GM, Honda and Nissan.
Also, several of the early systems utilized fixed code transmission. Suppliers such as Remotes Unlimited can clone fixed code remotes and often have a working master they can use if your original remote is lost or broken.
So, don’t give up on replacing that Isuzu remote until you have done some searching or spoken to an expert.