Try selenium diluted 1 part toner to 3 parts water. The final result will depend on how much information there is on the underexposed negative. There are other methods such as chromium which I believe is still available from Fotospeed, not too far away from where you are. There is a formula for mercury intensifier that is very good in that it is will certainly intensify very thin negatives. The down side is that it will increase the grain size and mercury is very difficult to obtain.