I do mods, and a lot of conversions to elsco guides. I haven't found anyone who makes specific conversion plates for different manufacturers. Last job I did I just ordered earthquake plates direct from elsco. They worked fine for going from US Elevator slide guides to elsco model b. I just welded the earthquake plates to the crosshead and the hole pattern is there for the elsco guides. Probably won't work in every case though, then there is always the local metal fabricating shop.
Cheers