The Celwave Duplexer model number says you have a UHF system.
The repeater:
EVX-R70
www.motorolasolutions.com
Shows that it is capable of analog or DMR digital.
SL300 is a DMR capable radio, and it shows analog capability.
Motorola support isn't going to be much help. About all they could do is direct you to a local Motorola shop that would look at your system and tell you which Motorola model radio you'd need to buy to work with that system. They won't suggest another brand that might save you money.
There's a lot involved in replacing the radios. They need to be programmed to work exactly the same as the originals and within the confines of the FCC license (if you are in the USA).
It won't be as simple as buying a radio off Amazon.
Unless you have all the details necessary to program a new radio, as well as the programming software/cables, you don't have much of an option.
You need to contact a radio shop that has the skills to do this correctly. That might be the same radio shop that set your system up. It is possible to lock some of these systems down with passwords, so random radio guy may not be able to do it.
Since this is primarily a hobby radio site, you'll probably get a lot of hobby oriented responses. You may get some that suggest you buy some cheap Chinese radios off Amazon/EBay and use those. Before you spend money on that, you need to understand that there are a ton of variables and randomly guessing the setup on the radios will not be easy. Also (if you are in the USA), there are FCC requirements that dictate what radios you can be using. The cheap $15 radios off Amazon/eBay rarely meet those FCC requirements.
If your employer is expecting you to do this without the proper training/equipment/knowledge (especially knowledge of the FCC rules), I'd recommend politely declining and giving them the contact information for a local radio shop.
As for the SL300's, you are probably looking at $300 each for replacements, maybe a bit less if you have a good shop.
There are less expensive DMR capable radios from other vendors. You pay a premium for the Motorola brand name on the current radios. But getting those from the same shop can save you a lot of headaches.
Unfortunately, once you let a radio shop in to set up a system like that, they kind of have you under their control. Unless you have a trained/experience radio guy on your staff (not a hobbyist), you don't have many options.