There are several inexpensive conversions for Dynamax fans and sevaral not so inexpensive ones as well. EJF has an adapter plate for about $60 which will allow the attachment of outrunners and inrunners. Mostly I have seen outrunners used, but there seem to be some "new kids" on the block so the motor choice is constantly evolving. A prop adapter and maybe a standoff and you're in business. You use the shroud and impeller from your existing fan. There are several threads on the Groups on conversions, or ask around if you're ready. Someone will chime in.
John