The compatibility very significnatly depends on the specific versions of your devices.
I would not even bother with the WRT54g -- even if you have a version that can theoreticaly run OpenWrt, the wireless performance will be so bad that it's not even worth it.
The Belkin "N300" is usually not a model number. What does it say on the bottom/back label for the specific model number?
I have an WRT 54GS that is still working and is used in my holiday home (crappy 10/2 Mb ADSL) but that is running DDWRT.,
Wifi is only G so max 54 Mb and when used as repeater you have to divide that between upload and download so maybe not worth the trouble.
Regarding the WRT54G is the 2.0, very very old. @egc which version of DDWRT are you using? The WiFi network where the device should be connected is very slow, I can imagine that even g (54Mbps) will be enough for now. @egc Can you install DDWRT via webGUI or do you need serial/telnet/ssh to install?
Divide by 2 if you're using it in a repeater type mode (i.e. wireless backhaul). Then divide by 2 again for an optimistic view of the best case real-world throughput. So figure you'll probably see less than 15Mbps.
And, now let's talk about this... v2 support OpenWrt 10.03.1 -- that firmware has a cumulative 13 years of security vulnerabilities and should not be used anymore, full stop. It's been EOL and completely unsupported for a really really long time, and you'll be lucky if anyone even remembers how to configure it because the syntax will be so drastically different than anything recent. Now, let's also consider that it may or may not even be possible to install relayd (space and/or package availability), and I'm not sure if that radio will support simultaneous sta and ap mode operation.
Seriously, it's not worth doing anythign with an 802.11g device these days.
'Similar to' won't tell us if your specific model is actually supported.
What does the box say on the outside? Particularly near the barcode. It may have a model number, but the product barcode could also be used to figure out the specific model/version.
I am on DDWRT build 52148 (which is from this year).
For further instructions see the DDWRT forum.
Read the Forum guidelines (first thread of the forum) which are very informative and post questions in the Broadcom forum, just as here people over there are always willing to help you.
Yup.. looks like it. So neither of these devices is suitable for OpenWrt... newer hardware will be necessary. Take a look at the table of hardware as you look for new device.