Hello,
The only track in Helsinki/Vantaa that is worth of a visit is Tattarisuo. You can find some pictures behind
this and a map where it is
here. There is also a karting track and the rc-car track is behind it. There is a "Nuorisoasian keskus" over there. If you know somebody who understands finnish
here is more descriptions how to get there.
Track is open monday-friday 12-20 and saturday-sunday 12-18. Practicing on the track doesn´t cost anything. If you drive your car on thursday there can be many electric cars (club night for those) with you and some of the drivers might not like it if somebody drives gas car (only a guess). The local club arranges club competitions for electric powered cars on 7.8. and 21.8. , and gas cars (200mm touring) on 5.8. and 19.8.
I know one (probably the only in Finland) shop that sells Mugen parts and it is only a mail-order shop so they usually send your orders in mail. Call there and ask if it is possible to retvieve goods from the warehouse. Shops website:
http://www.rc-kauppa.com/ and phone number is +35850-382 5988. The shop you talked about might be Hobby Point or Pro Hobby. There are also two (maybe even more than two) more shops here in Helsinki;
Tietonikkari and
RC-Superstore.
If you don´t understand something ask more.
P.S. Have a good vacation in Finland.
edited: When you this kind of sign you are at the destination. Picture:
http://koti.welho.com/tkauppin/tattarisuo/radalle6.jpg