Which tonearm for vintage Micro Seiki BL-91

I have to agree with Bpoletti, the MA-505 is probably the best overall match for this deck; the MkII for "standard" cartridges or the MkIII version if you want to use a high compliance cartridge. The BL-91 is a little resonant, so IMO a dynamically balanced tonearm will work better than a static arm. Also, in order to mount a VPI arm you are going to have to drill a blank mounting plate. The geometry of the deck will limit you to a 10" or (possibly) a 10.5" arm.

I have setup many MA-505s, and if you take care to match the right cartridge and take a little time to set up the arm properly these tonearms can really sing. I have had very good luck with either the Benz Ace and Glider LOMC carts with the MA-505 MkII, but there are several other cartridges that will work well with this arm.

Tgaral, please don't misunderstand my previous comments. The BL-91 is a great deck. But turntable technology (or our perception of such) has changed a bit since the late 1970s. The BL-91 is constructed of veneered plywood. This was quite good in the late 1970s, and was considered to be non-resonant. Improvements in materials and construction techniques have led to significant changes in plinth technology over the last 25 years or so.

Remember, the original (standard) tonearm for this deck was the MA-707, which is rather low mass unit. I believe that the MA-505 will provide a much better sonic result withour breaking the bank. Can you do better? Probably, but what is it worth for the small additional gain? (this, of course, being the original audiophile conundrum)

If I read your last note correctly, you are being offered an MA-505 for $300? If this is a "real" MA-505 (and not a re-purposed DD-40 variant) this is a very good price. Is it a MkI or II (S-shaped) or a MkIII (straight) model?

Regarding your questions about additional damping for the BL-91, frankly I wouldn't bother. You might make some minor improvements but in doing so you may jeopardize the resale value of the unit. Unless you hear a problem when played, I would just leave it alone and enjoy it for what it is: a very nice vintage turntable.

No VTA on the fly though so a bit worried about that

Unless you have so much time on your hands that you can afford to (or are anal enough to want to) reset VTA for different LP thickness, VTA on the fly is probably only really necessary if you are going to be swapping cartridges often.