Hi

I too have toyed with a go at .15 speed.

There is a commonly available motor ( ebay etc ) that is a copy of the Rossi .15 speed motor.
It goes under the names of MDS or summat like "UKATAM".

They usually come with a tuned pipe and are very fast, (28,000 rpm on a 6x6 or thereabouts iirc ) however take care if you are looking at bidding or buying one as there are plenty of new ones around and they can be got really cheap ( because there is little demand ) and some folks don't realize that and get carried away because it is a "racing motor".

I got mine new in the box for £20 with pipe, book etc.

Fitted with a Rossi style "button head" which lasts for ages and can easily be got anyhow

Let us know how you get on...