I do not think that you understand what you are doing.
Firstly you have to make sure that the given model are precached.
I do not see why would you want to change bomb targets' models. Be aware that its entities are invisible so your changes will not take any actual visual effect anyway.
EngFunc_SetModel accepts an entity index, not its classname. Either way such usage is very confusing. You have to read the fakemeta_const.inc before you going to start scripting using fakameta module.