The following thread may be helpful.
https://www.rc10talk.com/viewtopic.php?t=28751
The download link in the last post of the thread above is not working. I will look into options to make the file available and post an update. For now, the following text may be helpful. Please note, my experience is limited to the CAT 2000 '98 but the kits of this generation had many parts in common.
The linked writeup describes what is required to use Menace driveshafts (U2695), rear hubs (U2688), and hexes (U2682) in place of the the CAT 2000 '98 rear driveshafts (U1915), rear hubs (U1843), and hexes (U1408). Though the Fireblade 2000 and the Fireblade USA had different rear arms (U1983 and U2088) than the CAT '98 (U1800), all three have the same rear driveshafts (U1915), hubs (U1843), and hexes (U1408) listed on the exploded diagrams. The Fireblade 2000 used steel diff washer carriers (U1891 and U1892) and the Fireblade USA used alloy diff washer carriers (U2006 and U2007).
Additional observations from the Schumacher exploded diagrams are as follows:
The Fireblade 2000, the Fireblade USA and the Cougar 2000 used the same rear hexes (U1408) and hub carrier bearings (U1439) .
The Fireblade 2000 and the Fireblade USA exploded diagrams list the U1843 hub carriers and the Cougar 2000 lists the U1431 hub carriers. Cross referencing the CAT 2000 (U1431 hub carrier) and CAT 2000 EC (U1799 -- should be compatible with the optional U1843 hub carrier) exploded diagrams indicates both were compatible with the U1510 axle and therefore should have the same dimensions required to fit the U1915 driveshaft axle.
The Fireblade 2000 and Fireblade USA use the U1992 diff gear and the Cougar 2000 uses the U1442 diff gear. All three exploded diagrams list the U730 (U1954 is the "Pro" option) thrust bearing, U1415 diff washers, U1419 bearings, and U1420 diff balls.
The Fireblade diff screw set part number is U1887 and the Cougar 2000 part number is U1417.
The Fireblade diagrams list U1923 flanges while the Cougar 2000 diagram lists U1423. Based on the parts pictured, the Fireblade's diff may be a different diameter than the Cougar 2000 diff.
The Fireblade 2000 rear arms are U1983 the Fireblade USA rear arms are U2088 and the Cougar 2000 rear arms are U1433.
Additional notes
The Cougar 2000 exploded diagram in the writeup shows U1413 driveshafts along with U1469 and U1470 diff washer carriers. It is not clear what the difference is between the U1469 and U1470 diff washer carriers and the U1891 and U1892 diff washer carriers. They could be different lengths.
It is not clear why Schumacher used the telescoping driveshafts (probably U1510 and U1511) and the corresponding carriers (probably U1504 and U1505) on some models and steel driveshafts on others. Perhaps someone can explain why.
The light alloy female and male outdrives U2006 and U2007 will get chewed up by the steel driveshaft pin of U3752 linked in the above posts. The compatible steel outdrive equivalents are:
U1891 Diff Washer Carr Male - SST
U1892 Diff Washer Carr Female - SST
I do not have measurements of the newer generation driveshafts (e.g. U3752 linked above) to confirm if there are more modern compatible alternatives to U1915. There are a variety of dimensions that need to match up for an alternative to fit.
Do you have a picture of these driveshafts used with a Cougar 2000? Based on the picture on Schumacher's site, the U3752 driveshaft appear to have a 2.5mm outdrive pin. The U1915 driveshafts had a much thinner pin that held the 2.5mm thick U1918 blades.
Also, the dimensions of the U3752 driveshaft appear to be different than the U1915 driveshaft.
