I think nimh is a bust as a straight lead replacement.
You have a battery ignition, as in not a magneto. Per this link
http://pages.prodigy.net/klricks/CM185T.html If you have access to a large capacitor or 8 in the junk pile you can try that in place of the battery, shut off the lights, and kick like mad till it starts, see if it runs ok (leave lights off initially). The caps go in parallel, you need at least 12v, and large.
You have a nice 48v bike project to look forward too also
(or 24v @ 13ah)