It's probably one of those things you'll have to try and see how it goes. Start off tiny - maybe 5 minutes - until you see how your body reacts. And don't do anything that would jar your head.

I bought a recumbent bike http://www.walmart.com/ip/Stamina-Ma...t-Bike/7680164 the first of the year (9 months pcs) and was able to use it as long as I don't let my pulse get too high. Some days I can walk now (1 year pcs).