My 420 just failed emissions and I need some advice. It has about 180,000 miles on it.

Here is the complete test (failed tests are in bold):

PASS - HC ppm reading was 63 of 103 on 25/25 and 34 of 107 on 50/15
PASS - CO % reading was 0.05 of .57 on 25/25 and 0.02 of 0.59 on 50/15
FAILED! - NOxppm reading was 2308 of 712 allowed on 25/25 and 3308 of 786 on the 50/15 test.
PASS - RPM reading was 1375 of a 2500 max on 25/25 and 1286 of 2500 on 50/15
CO+COe% reading was 14.6 of 6.0 min on the 25/25 and 14.1 of 6.0 min on the 50/15

The Catalytic Converter was read as: Present

Fuel Cap Test 1: Fail

I'm being told I need to replace the Catalytic Converter and was wanting some advice before I jumped into that. If a converter is needed are there any good universal afterkmarket options?

Thanks in advance!