First - Welcome to the Site!!
If you were able to download the service manual I think you should be able to download that file. If not you might need 5 likes.
This is essentially what is in that file. As you can see it does not have a size for the 450.
Designation Tray Drive Belt Sled Drive Belt Comments
DTL-50 0.040"sq x 4.00" 0.040"sq x 4.00" Digital Time Lens
DTL-100 0.040"sq x 4.00" 0.040"sq x 4.00" Digital Time Lens
DTL-200 0.040"sq x 4.00" 0.040"sq x 4.00" Digital Time Lens
DTL-200mk2 0.040"sq x 4.00" 0.040"sq x 4.00" Digital Time Lens
MD/A-420 made in china
MD/V-500
MV-5
PSD-36b N/A (magazine) N/A (worm gear)
changer, 10 disc magazine
SD/A-350 changer, 5 disc carousel
SD/A-360 tray in/out 0.065"sq x 4.60"
tray rotate 0.065"sq x 5.00"
changer, 5 disc carousel
SD/A-370 changer, 5 disc carousel
SD/A-390t 0.075"sq x 3.70" 0.075"sq x 3.70" tubed, changer, 5 disc carousel
SD/A-410
SD/A-450
SD/A-490t 0.046"sq x 2.40" N/A (worm gear) tube reference player
TL-3100 0.040"sq x 4.00"
TL-3200
TL-3220
TL-3300
TLM-3600 N/A (magazine)
N/A (worm gear)
changer, 10 disc magazine
Does the belt still exist in one piece?