electric heaters for spa can be installed on a water re-circulation system feeding the soaking tub

Thank you for your interest in our japanese bathtubs.
You should use equipment available in your area.

I would recommend to contact a SPA equipment dealer/installer near you.
An ofuro can be treated as a regular spa/hot tub.
The equipment sizing will depend on the capacity of the tub.

I found this compact heater without tank (post-heating).
http://www.pahlen.com/our-products/heating-electric-heaters/electric-swimming-pool-spa-heater.html

Of course it should be coupled with a small pump for recirculation and with a cartridge filter if you wish.

Since ofuros are made of solid wood, you can open holes for inlet/outlet almost everywhere.

Please let me know of any other question/problem.

Best regards,