Use the service check connector to determine what the stored error codes are in the ABS system. The procedure for code retrieval along with a list of the error codes are described in the brake section of the service manual
https://www.nsxprime.com/FAQ/Reference/1991_svcman/1991servman.pdf
The error codes and the service manual will allow you to confirm the nature of the problem.
The most reliable source for NSX parts is probably Amayama. Amayama still shows the pump assembly as being available. However, it is subject to updates and stuff can disappear.
Absアキュームレーター for Honda NSX 1 generation 09.1990 - 11.2001 - Honda Car and Auto Spare Parts - Genuine Online Car Parts Catalogue - Amayama
Use your vehicle VIN to confirm that you are looking at the correct parts page - I am guessing that you might have a right hand steer car which has some differences than left hand steer cars. If you had the failure diagnosed by a Honda dealer, the other option is to ask them for the relevant part numbers. You can do a part number search on Amayama to determine if the part appears to be available. Unlike the Honda / Acura critical parts order process, parts delivery from Amayama is fairly quick if the part is available.
You can completely delete the ABS system. There are threads discussing that on Prime. You can also update the ABS modulator to the more modern / compact / generally better in every way modulator that was used on the NSX starting in 2000. You can also install the modulator from a Honda S2000 which is similar to the 2000+ NSX modulator. The advantage of the S2000 modulator is that you can find salvaged units on EBay and other places for a lower cost than than a new NSX 2000+ modulator. The retrofit requires some fabrication. You can find threads on Prime describing the S2000 and NSX 2000+ modulators.
Science of Speed sells a complete package for upgrade to the later NSX modulator. Not inexpensive.
ScienceofSpeed NA2 ABS System Upgrade - Brake Accessories & ABS System - Brake Products - NSX, 1991-05
It might be cheaper to purchase the parts individually from Amayama and source the wiring harness from Science of Speed.