If the spec of your bike is the same as
this, then I suspect your existing BB is JIS square tapered, because afaik that Sugino chainset is square tapered.
To answer your question, XTR/XT compatible/matched square tapered BBs have not been made for many years, it could be hard to find one in good condition, let alone one that has the requisite shell spec and spindle length for your existing chainset.
As to whether one should/could replace the whole chainset/BB with current XTR/XT components (all of which are integrated chainset-BBs), it can get complicated since the bike appears to have road STI, but the front mech is supposedly a FR-M4503 which doesn't appear to exist in Shimano's product line. This doesn't surprise me because road STI won't pull enough cable for a typical FD-4503 front mech, so it might be a "special". In any case it must have a profile for 48/50T large ring, and if a 45mm "road" chainline then it would not be an ideal front mech for any current XTR/XT chainset since they all have large rings up to only 44T and chainlines close to 48/49mm. If one were to change the front mech for a mtb front mech as well then one will run into cable pull incompatibility issue with the STI. Complicated, but perhaps you get the gist of potential knock-on effects of change.
The simplest way forward therefore is to replace the BB by an off-the-shelf BB on a like for like basis in terms of shell and spindle size, or the chainset/BB combo by another road triple with similar if not identical large ring and range. In terms of quality BBs I think
this shows as good a range as any, with perhaps the exception of missing the Royce ones.
Hope it helps.
