How to fit an ex-wald distribution model with R?
This seems promising. I haven't used it

Heathcote, A. (2004). Fitting Wald and ex-Wald distributions to response time data: An example using functions for the S-PLUS package. Behavior Research Methods, Instruments, &amp; Computers, 36(4), 678-694.
http://www.tascl.org/uploads/4/9/3/3/49339445/26_.pdf

It does include s-plus code in a supplement to the journal article:
https://link.springer.com/article/10.3758%2FBF03206550

Often S-plus can be used in R, sometimes with some adjustments.